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Seismic-stratigraphic interpretation has become an important element of exploration in basins with limited well control. This new direction in exploration imposes new responsibilities and qualifications on both the geologist and the geophysicist. Two general approaches are developing in response to exploration requirements--a physical approach involving processing and synthetic modeling, and a seismic-stratigraphic approach involving a new application of traditional facies geology.
Seismic-stratigraphic analysis of Brazilian offshore basins permits the development of approaches and concepts that can be applied to other basins. Analysis involved development of seismic-stratigraphic framework, interpretation of reflection patterns, chronostratigraphic correlation, mapping seismic-stratigraphic (depositional system) units, synthesis of depositional and facies interpretations, and, in many cases, strategic mapping of specific facies. Within Brazilian offshore basins, three principal depositional systems are recognized--delta and fan delta, carbonate platform and shelf, and slope. By integrating seismic and limited well data, it is possible to recognize on reflection seismic sections: (1) three deltaic facies--prodelta and distal delta, front or barrier; delta, front or barrier; and alluvial and delta plain; (2) two fan-delta facies--proximal and medial fan, and distal fan and prodelta; (3) three shelf and platform facies--neritic; reef, bank, shoal, and shelf edge; and submarine canyon fill; and (4) three principal arrangements of slope facies--offlap, onlap, and uplap.
Integration of conventional and seismic-stratigraphic analyses permits recognition of five fundamental types of rift and pull-apart basins in Brazilian offshore areas: early rift-fault basin, post-rift clastic basin with salt tectonism, post-rift basin with stable carbonate platform, pull-apart basin with passive clastic-carbonate offlap and onlap deposition, and pull-apart basin with deltaic sedimentation. Seismic-stratigraphic analysis permits extrapolation of limited well data to predict depositional systems tracts, tectonic elements, principal depositional modes, and source area and drainage characteristics. Similarly, the geologist can predict reservoir type and spatial distribution, stratigraphic and structural trap possibilities, and source bed and seal potential.
Stratigraphic interpretation of seismic data has become an increasingly important element in exploration during the past 10 years, particularly in offshore areas. Exploration in frontier or poorly known continental shelf areas, where well control is unavailable or limited, has required a greater degree of stratigraphic interpretation of existing geophysical data. Integration of geologic and geophysical methods in response to exploration requirements has imposed some significant changes in approach and emphasis in basin analysis. Similarly, significant changes are occurring in the companion field of exploration geophysics. Probable reasons for the rapid emergence of seismic-stratigraphic capability at this time are (1) the dramatic advances in computer technology and seismic data acqui ition, and (2) a similar breakthrough in basin analysis during the past 10 to 15 years resulting from the development of Holocene depositional models that advance the understanding of depositional processes, depositional environments, and facies interpretations of ancient deposits. The fortuitous and parallel advance in geophysics and basin-analysis concepts is providing some very useful exploration tools.
This new direction in exploration imposes new responsibilities on both the geologist and the geophysicist. Teams composed of both specialists are common today in many exploration groups, but it is increasingly obvious that a need exists for explorationists who possess expertise both in geophysics and facies analysis. Certainly, successful explorationists, geologists, and geophysicists will be involved in the emerging field called seismic stratigraphy.
Emerging from the seismic-stratigraphic "revolution" are at least two areas of specific interest and application: (1) a physical approach aimed at greater and more accurate discrimination and synthetic modeling of lithic composition, fluid content, and other similar properties, utilizing computer analysis of velocity, amplitude, and cycle
parameters, etc., and (2) a stratigraphic-facies approach using reflection sections and density or sonic-log data to invoke facies interpretations and to integrate, spatially and chronologically, the depositional systems which fill basins.
Reflection seismic sections provide the experienced basin analyst with a vehicle for applying state-of-the-art facies geology to basins with limited well control. Although seismic sections have been used for many years for structural mapping and interpretation, the maximum stratigraphic significance and value of the section can be realized by appreciation of the facies fabric of basins of different tectonic settings. There are obvious pitfalls in inferring stratigraphic facies from seismic data, just as there are from making these inferences from other remotely sensed and indirect data such as well logs. Consequently, the successful application of seismic stratigraphy will involve the joint participation of geologists and geophysicists, or individuals adequately experienced in both ar as.
This report provides a general perspective of seismic-stratigraphic interpretations that have been made in offshore Brazilian basins. Interpretations have involved application of facies geology integrated with analysis of seismic-reflection data. Interpretations were made jointly by geologists and geophysicists.
The report is not a catalogue of seismic-facies patterns, but rather a general guide to an approach that organizes available seismic-stratigraphic data into an integrated view of a sedimentary basin. Neither is the report intended to convey the geology of any specific offshore Brazilian basin, but rather we are attempting (within proprietary constraints) to present some principles that were developed during detailed basin analysis using suites of well logs, paleontologic information, cores, and seismic data. Furthermore, we hope to illustrate the applicability of seismic-stratigraphic methods in frontier basins and further provide an independent example to compare with the studies in this volume by geologists and geophysicists of Exxon, U.S.A. (Vail et al, this volume).
Knowledge of facies geology is a fundamental requirement. Regional and local seismic reflections provide a seismic-stratigraphic framework. Distinctive seismic reflection patterns and available well data were the basis for facies interpretations and mapping.
Although numerous abstracts and reports have been published on the physics of seismic-stratigraphy, only a few abstracts are available that specifically address the stratigraphic and facies aspects of seismic interpretations (e.g., Vail and Sangree, 1971; Vail, Mitchum, and Thompson, 1974; and Sangree and Widmier, 1976). A recent report by Sangree et al (1976) summarizes most subjects previously covered by oral presentation. Reports by the above authors and others are included in this volume, and herein are referred to collectively as Vail et al. Reports by these geologists and geophysicists offer a comprehensive and critical analysis of seismic facies interpretation. In our Brazilian studies, presented as a series of unpublished company reports prepared for Petrobras (Petroleo Brasil iro, S.A.) during the period from 1973-1976, we independently reached many similar conclusions based on data from different basins. Although differences in approach and interpretation exist between the studies by Exxon and Petrobras, the observational differences are minor and, in part, may result from differences in the basins that were studied. Principal differences are interpretive and involve the use of relative sea-level control by Vail et al to explain formation of "submarine canyons" and onlap slope deposition. Variations in nomenclature occur, and will be equated where possible.
Extensive literature is available on facies interpretation, both Holocene and ancient: LeBlanc, 1972; Shelton, 1973; Reineck and Singh, 1973; Fisher and Brown, 1972; and others.
Source of Data
Since 1973, analyses of offshore Brazilian basins for Petrobras provided an opportunity to apply depositional-systems (or facies) analysis to large basins with limited well control. Seismic reflection data provided the principal source of subsurface information which had to be integrated with information derived from well logs, samples, and rare cores.
Illustrations of seismic sections used in this report are generalized from actual sections, because no seismic sections are available for publication. We are keenly mindful of the constraints and limitations this imposes on our presentation. Although actual records would be ideal to illustrate the ideas presented in this report, it is hoped that readers can follow the presentation adequately using diagrammatic illustrations which show general attitudes and continuity of key reflections. The examples are a composite, selected from studies of most offshore Brazilain basins: Serigipe-Alagoas, Espirito-Santo, Mucuri-Cumuruxatiba-Jequitinhonha, Potiguar, Foz do Amazonas, Santos, and Barreirinhas basins. Because of proprietary constraints, specific aspects of the basins such as vel cities, well control, paleontologic interpretations,
and other factors are omitted in favor of a presentation of general reflection patterns and inferred depositional interpretations.
CONCEPT OF DEPOSITIONAL SYSTEMS
Stratigraphic interpretations utilizing seismic reflection data should be firmly based on an adequate understanding of the three-dimensional arrangement of lithofacies and their integration into depositional systems. It is essential, therefore, that the seismic stratigrapher understand depositional processes and facies models and conventional subsurface geology, as well as seismic geophysics, before attempting to infer stratigraphic and facies relations from a seismic section.
We believe one of the most useful concepts in seismic-stratigraphic analysis is that of "depositional systems." Fisher and McGowen (1967) defined depositional systems as three-dimensional assemblages of lithofacies, genetically linked by active (modern) or inferred (ancient) processes and environments. A depositional system is the stratigraphic record or analog of deposition within the myriad environments that constitute river, delta, barrier island, shelf, and slope systems, among others. The fundamental unit of the depositional system (as used in this report) is the lithofacies, a three-dimensional sediment or rock body bounded by depositional (or erosional) surfaces whose genesis is inferred from the interpretation of sedimentary structures, textural variations, bedding characteris ics, internal and external stratigraphic relations, paleontology, and association with adjacent facies. A depositional-systems approach has been used by various workers using conventional subsurface data (Fisher and McGowen, 1967; Fisher, 1969; Brown, 1969; Guevara and Garcia, 1972; and Erxleben, 1975). A wide knowledge of various depositional systems permits prediction of component facies composition, geometry, and distribution using limited data. More importantly, an understanding of the spatial arrangement of depositional systems within various types of basins provides a fundamental tool in the stratigraphic interpretation of seismic reflection sections. When the spatial arrangement of facies or systems within various types of basins is understood, it is possible to infer with greater confidence stratigraphic relations and depositional patterns from subtle variations in reflection attitudes and continuity.
Contemporaneous depositional systems can be linked to produce what may be called a "systems tract." For example, fluvial, delta, shelf, and slope systems may be intergradational and, in part, contemporaneous. In addition, the tract defines paleoslope from basin margin to deep water. A basin is filled by deposition within a variety of systems tracts which evolve through time as tectonics and source areas change. Significant changes in the style or mode of deposition in the basin are commonly marked by regional, sometimes basinwide, seismic reflections (conformable or unconformable boundaries). The regional reflections constitute isochronous surfaces within a basin, except where they represent unconformities. The resulting reflection-bounded units composed of contemporaneous depositiona systems (systems tracts) are herein called "seismic-stratigraphic units" (depositional and cyclic sequences, Sangree et al, 1976). The seismic-stratigraphic unit is the principal element of the seismic-stratigraphic framework of a basin. Recognition and delineation of principal and minor seismic-stratigraphic units will be discussed later.
EXAMPLES OF CONVENTIONAL BASINAL ANALYSIS
The general arrangements of facies and depositional systems within two types of basins, the Gulf basin of Louisiana and Texas and the Eastern Shelf of the Midland basin of Texas, provide examples of basin-fill style within a rapidly subsiding, oceanic-margin basin underlain by salt, and within an intracontinental basin that exhibited tectonic stability, respectively (Figs. 1, 2, 3). These examples (where the writers have had personal experience) are based principally on interpretations of conventional subsurface data in basins that are in mature stages of exploration. Many reports cover all aspects of these basins; references are not included, but the reader is referred to standard bibliographic sources. The two contrasting basins demonstrate similarities and differences in type and d stribution of depositional systems within basins of significantly different tectonic style.
Basins such as these have been studied intensively, and constitute excellent "models" that may aid in the interpretation of other less-explored basins. Basins throughout the world can be grouped into a number of fundamental types (Klemme, 1971) which exhibit similar systems tracts and depositional history.
Gulf Basin of Louisiana and Texas
Although the early history of the Gulf basin is not well known, it was apparently marked by rift-basin tectonism characterized by fan, fan-delta, and salt deposition (Fig. 1A). Subsequently, a series of depositional systems tracts evolved, composed of Jurassic and Cretaceous shelf-platform carbonate, shelf-edge reef, and slope systems, and Cretaceous and Tertiary fluvial, delta, and slope systems.
Fig. 1. General characteristics of facies in northwestern part of the Gulf basin. A. General cross section of Gulf basin showing principal depositional systems (Modified from Lehner, 1969). B. Dip cross section of Eocene delta system illustrating internal facies composition (after Fisher and McGowen, 1967). C. Regional map of inferred Hackberry (Oligocene) slope system (adapted from Paine, 1968).
Fig. 2. Distribution of principal depositional systems in Fort Worth-Foreland basin and Midland basin during Late Paleozoic. A. General dip cross section illustrating change in depositional style during tectonic evolution of the basins (after Brown et al, 1973). B. Block diagram of Eastern shelf of Midland basin during Late Pennsylvanian time when deltaic, shelf, and slope systems were operative (after Brown, 1969).
Fig. 3. General characteristics of Upper Pennsylvanian-Lower Permian facies in eastern part of Midland basin. A. Regional dip section showing deltaic, shelf, and slope facies (after Brown, 1969). B. Internal facies relations in a Pennsylvanian-Permian delta system that prograded across and intertongued with a carbonate shelf system (after Brown et al, 1973). C. Reconstruction of slope deposition by submarine fans during Late Pennsylvanian and Early Permian (after Galloway and Brown, 1972).
Tertiary deltaic deposition involved both river- and marine-dominated delta and associated slope systems, which constitute the largest volume of basin fill. Marine-dominated delta systems, such as the Oligocene Frio system, contrast with the Wilcox type by exhibiting extensive marine sandstone facies of barrier island origin. Delta systems in the Tertiary of the Gulf basin are characterized by thick, superposed facies, reflecting a balance between rates of deposition and subsidence (Fig. 1B). Growth faulting and salt or shale diapirism are also commonly associated with these thick delta and associated slope prisms.
Slope systems in the Gulf basin are generally beyond the drill, but several examples, such as the Oligocene Hackberry slope system of Louisiana and Texas (Fig. 1C) and the Yoakum system associated with the Upper Wilcox Group (Eocene) of Texas, provide some insight to the character of Tertiary deep-water facies. Slope systems in the Gulf basin were deposited in close association with contemporaneous salt diapirism, probably mobilized by the massive sedimentary load imposed by the larger prograding delta and associated slope systems. Plio-Pleistocene deltas have similarly mobilized salt tectonism in the modern slope of the Gulf of Mexico.
Eastern Shelf of the West Texas Basin
The eastern flank of the West Texas basin of Late Pennsylvanian and Early Permian age (Figs. 2, 3) provides a view of another type of basin where the fill is generally understood from conventional subsurface methods. The Fort Worth basin (Fig. 2A) was the site of westward prograding Late Mississippian and Early Pennsylvanian depositional tracts composed of fan, fan-delta, and slope systems. To the west (on the Concho Platform) were extensive carbonate environments with eastward-facing shelf edges. This fan, fan-delta, slope, basin, and carbonate platform systems tract evolved into Middle Pennsylvanian fluvial, delta, and carbonate bank tracts and ultimately into a Late Pennsylvanian-Early Permian fluvial, delta, carbonate-shelf, and slope tract (Fig. 2B). This final depositional style (Fig. 3) was responsible for filling much of the West Texas basin. Deposition was concluded by the Middle and Late Permian tidal and evaporite depositional episodes. Because the intracontinental basin subsided slowly, deltaic depositional rates far exceeded subsidence rates, resulting in widespread, thin cyclic sequences (Fig. 3B). Slope systems progressively filled the basin by deposition of a series of offlapping wedges of deeper water deposits (Fig. 3A, C).
The brief review of the Gulf and West Texas basins emphasizes a very important concept for either conventional basin analysis or seismic-stratigraphic analysis, which is that basins are filled principally by basinward accretion of the sedimentary prism (Fig. 4). The seismic stratigrapher must be aware that most basins "fill" laterally, rather than "fill up" through time. The degree of superposition of these wedges is controlled primarily by rates of basin subsidence, compaction, and salt or shale flowage. Although vertical aggradation occurs in many shelf environments and within some delta and slope environments, progradation of deltaic and slope systems occurs primarily along depositional surfaces that are inclined generally from 1 to 5°. These paleodepositional surfaces constitu e isochronous surfaces at any instant in geologic time. Relief of the depositional surfaces is greatest in the slope
Fig. 4. Diagrammatic representation of isochronous lines (surfaces) in hypothetical basins, based upon regional and localized reflection surfaces. A. Progressive filling of a basin by shelf, slope, and basinal deposition. B. Fan-delta, carbonate-shelf, and slope progradation with periodic continental rise onlap. C. Delta-slope progradation (offlap).
environment and least in the delta environment (Fig. 4B, C). An appreciation of the configuration of various modern depositional surfaces or profiles is invaluable when inferring depositional topography from the configuration of reflection attitudes on seismic sections.
With this review of two types of basins for which extensive, conventional subsurface data exist, we will now focus on lesser-known basins where principal data consist of seismic records.
Following data acquisition and processing, several basic steps or factors are involved in a seismic-stratigraphic analysis: (1) recognition of regional (and certain minor) reflections or reflection discontinuities that subdivide the basin fill into seismic-stratigraphic units which represent distinctive episodes of deposition, constituting the basis for a stratigraphic framework of the basin; (2) integration of the seismic data (e.g., velocity data, reflection strength, continuity, attitude and evident anomalies) with any available well data (cores, logs, paleontology); (3) correlation of paleontologic data with regional and minor reflections to develop approximate chronostratigraphic correlation within the basin; (4) isopach mapping of principal and minor seismic-stratigraphic units sing velocity-analysis data correlated with available well data; and (5) synthesis of a systems (or facies) tract and depositional models for each seismic-stratigraphic unit.
Development of a seismic-stratigraphic framework is similar to construction of conventional stratigraphic control using traditional subsurface data, except that seismic lines provide the opportunity for continuous correlation throughout the seismic grid. The writers agree with Sangree et al (1976) that reflections represent depositional surfaces except where they coincide with unconformities. Reflection-defined depositional surfaces constitute isochronous surfaces; the more widespread, continuous, reflections are fundamental stratigraphic markers within a basin (Fig. 5D). Regional reflections such as R1-R5 in Figure 5D bound principal seismic-stratigraphic units, commonly composed of two or more depositional systems; for example, fan-delta, carbonate-shelf, and s ope systems constitute a tract of facies deposited within contemporaneous environments, extending from continental areas to deep water. The depositional history and facies composition of the total basin can thus be determined by sequential analysis of each individual seismic-stratigraphic unit in the basin.
The regional reflectors that separate seismic-stratigraphic units generally represent significant changes in reflection coefficient at the interface between: (1) thin, widespread marine transgressive facies and subjacent deltaic clastic facies, and (2) contemporaneous blankets of thin, widespread, hemipelagic muds and subjacent submarine fan complexes of the continental slope. These reflections commonly mark a hiatus in normal depositional rates within the basin, either because of eustatic rise in sea level or regional subsidence.
Inclined reflections commonly terminate downward and generally basinward along the upper surface of a regional reflection. This has been called "baselap" by Vail et al (this volume). This phenomenon marks the initial progradation or offlap of deltaic or slope facies over a transgressive shelf or onlapped shelf (respectively) of a subjacent seismic-stratigraphic unit. Consequently, each unit represents a cycle of progradation and transgression.
Within the principal seismic-stratigraphic units, lateral boundaries between contemporaneous depositional systems are generally distinctive, but gradational. Consequently, reflection boundaries such as those that separate contemporaneous fan-delta and shelf systems are normally gradational and intertonguing. However, systems boundaries such as those between horizontally stratified carbonate shelf systems and subjacent, inclined-slope systems are not so distinct; reflections may be slightly to prominently discordant--called "toplap" (Vail et al, this volume).
The general procedure for developing a basinal framework involves: (1) recognition of regional unconformities distinguished by onlap, baselap and/or truncated reflections; (2) extrapolation of these reflections throughout the basin into areas where the reflection coincides with conformable depositional surfaces; (3) recognition of intertonguing or discordant relations between minor, contemporaneous seismic-stratigraphic units such as shelf and slope systems or fan-delta and shelf systems, respectively; (4) recognition of minor discontinuities in reflection attitudes such as exhibited by onlapping reflections in minor continental rise units; and (5) recognition of minor erosional unconformities such as those which occur at the base of submarine canyon-fill deposits. These critical refl ctions are then correlated and traced through every dip and strike seismic line in the basin. As the study proceeds, other less prominent reflections may be recognized and traced, especially those that subdivide minor episodes of marine transgression over deltaic facies or slope reflections that mark a lengthy period of hemipelagic deposition.
The resulting seismic-stratigraphic framework may resemble the example shown on Figure 5D. On this framework, other more specific facies interpretations from well and reflection analysis will later be superimposed. Isopach maps can be constructed from the travel time and velocity values for each seismic-stratigraphic unit in the basin. The framework also provides the basis for chronostratigraphic correlation, calibrated with biostratigraphic data available from limited wells.
Interpretation of Reflection Signatures
Because reflections can not be directly interpreted in terms of lithofacies, it is possible, using limited well data, to make some surprisingly accurate estimates of facies type and composition, by analogy with other basins where extensive well control is available. Naturally the more experience the geologist has, the greater is his probability of an accurate interpretation. Similarly, the more well control that can be tied into the seismic framework, the better the odds become for valid interpretation.
Sangree et al (1976) described a wide variety of reflection patterns and respective sedimentary facies from the Pleistocene of the northern Gulf of Mexico. Papers by Vail et al (this volume) also provide a comprehensive classification of reflection patterns. The writers will apply similar terminology, but some variations will occur that should be obvious to the reader.
Reflection attitude, configuration, continuity, and amplitude, when combined with external geometry of the signature and spatial association with other signatures, as noted by Sangree et al (1976), provide a range of reflection pattern combinations that may permit interpretation of the facies from the seismic record. In some instances, zones of distinctive diffractions and sideswipe may prove to be indicative to certain relatively unique velocity variations related to facies geometry rather than of structural origin. Newer lines shot with higher energy sources (such as the air gun) may significantly improve the quality of subtle reflections.
Rather than interpret the Brazilian facies using a geometric classification scheme (like the one applied by Sangree et al, 1976; and by Vail et al, this volume), we interpreted the seismic facies signatures within the context of depositional systems. Most geologists and geophysicists are faced with evaluating large, sparsely drilled basins. This was true of our Brazilian experience, and we believe that under these kinds of exploration imperatives, the best analysis will involve the closest integration of traditional depositional systems analysis and seismic-stratigraphic information. We used the seismic responses to extend and improve traditional basinal analysis interpretations, using available well data sometimes limited to 5 or 6 wells within 200,000 sq km. This combination of seis ic data and depositional (process) analogues provides a very reliable tool for facies interpretation, as shown by subsequent drilling.
By assuming that seismic reflections are isochronous surfaces or unconformities, it is possible to establish the relative order or sequence of stratigraphic units in a basin (Fig. 4). The chronologic sequence of depositional and structural events thus can be determined accurately. Where these reflections are calibrated with biostratigraphic data from wells in the basin, the seismically based stratigraphy can be assigned to standard time-stratigraphic units (systems, series, stages). In this manner, limited paleontologic information can be applied throughout the basin.
Conformable, regional, seismic reflections which bound the principal seismic-stratigraphic units in a basin (Fig. 4) serve as approximate time-stratigraphic boundaries. However, within each seismic-stratigraphic unit, minor isochronous surfaces may be complexly arranged. For example, units bounded by prominent regional reflections may be composed of deltaic facies which internally exhibit complex offlapping reflections (Fig. 4C). Where nondeposition occurred during a given time interval, two or more reflections will merge (Fig. 4A, B). Reflections may also merge along unconformable surfaces, such as those developed during coastal onlap (Fig. 4B).
Very elaborate time-stratigraphic subdivisions can be delineated in basins using seismic-stratigraphic criteria. This aspect of analysis permits very accurate correlation, and provides the basis for precise determination of the chronologic order of geologic events within the entire basin.
Mapping Seismic-Stratigraphic Units
Where a basinwide seismic-stratigraphic framework has been established, and regional bounding reflection surfaces have been traced throughout the seismic grid (Fig. 5D), it is then possible to map the various seismic-stratigraphic units and their component depositional systems (Figs. 5, 6). Travel time can be converted to thickness at selected shot points by using well and velocity data; and contouring, with reference to the seismic section (e.g. faults, shelf-edge positions, submarine canyons), permits a basinwide picture of each seismic-stratigraphic unit and most component depositional systems.
Fig. 5. General isopach patterns of seismic-defined depositional systems and their integration into a hypothetical, basinwide, seismic-stratigraphic framework. A. Seismic-stratigraphic isopach map of submarine canyon system. B. Seismic-stratigraphic isopach map of submarine fan complex (cone). C. Seismic-stratigraphic isopach map of deltaic and associated slope systems. D. Hypothetical seismic-stratigraphic framework integrating available wells and seismic data. On cross sections, terrigeneous clastic facies shown schematically by lines representing seismic reflection attitude and continuity.
Fig. 6. General isopach patterns of seismic-defined depositional systems. A. Seismic-stratigraphic isopach map of a fan-delta system. B. Seismic-stratigraphic isopach pattern of a shelf carbonate system. C. Seismic-stratigraphic isopach map of a slope system. D. Seismic-stratigraphic isopach map of fan-delta and slope systems within rift basin. On cross sections, terrigeneous clastic facies shown schematically by lines representing seismic reflection attitude and continuity.
Within the Brazilian basins, very distinctive (and predictable) isopach patterns characterize the various depositional systems that compose each unit. The isopach map of a delta system, that prograded across a continental shelf and over a contemporaneous slope system beyond the shelf edge (Fig. 5C), clearly delineates a subjacent shelf-edge position, principal dip-oriented depositional axes, and a distal growth-faulted complex. Seismic-stratigraphic isopach maps of shelf and platform carbonate systems (Fig. 6B), though, exhibit distinct isopach patterns that outline elongate trends parallel with the relict shelf edge, which probably represent reef or bank facies. Slope systems display isopach patterns that pinch out landward near the relict shelf edge (Fig. 6C) and exhibit exceedingly thick, closed contour values in areas of persistent submarine-fan deposition. Traced far into the basin, the slope system again thins into abyssal facies. Where slope systems were supplied with sediment via submarine canyons (Fig. 5A), the slope systems will extend landward into the canyon and pinch out in the relict shelf system. Fan-delta and associated slope systems that prograded into rift basins clearly outlined the basin geometry and may thicken toward contemporaneous faults. If the rift basin opened into the oceanic basin, the systems would thicken abruptly where they prograded into deeper water (Fig. 6D).
By mapping units defined by differences in reflection attitudes, continuity, or similar variations, it is possible to delineate a variety of specific facies (see later discussion). For example, by mapping the thickness of chaotic onlap reflection units within a slope system, fan-shaped isopach patterns emerge which define the limits of individual submarine fan complexes, or fan cones (Fig. 5B). Similarly, the mapping of units characterized by layered, but highly erratic, reflections that intertongue with layered reflections exhibiting extreme continuity, produces an isopach map of individual fan-delta lobes that prograded across and intertongued with a limestone shelf (Fig. 6A). Mapping of specific facies or individual depositional elements such as the submarine fan or fan-delta lobe, requires the recognition and delineation of relatively subtle reflection variations. With experience, nevertheless, the stratigrapher may gain insight to facies relations and distribution of economic significance. Criteria that help to identify relatively specific facies by seismic reflection patterns and relations may become subjective. The regional mapping of principal seismic-stratigraphic units is more objective but only provides a general guide to the potential reservoir facies.
A final phase of analysis involves a synthesis of conventional subsurface data and inferred seismic-stratigraphic information. The spatial arrangement and chronologic order of facies within each depositional system can be estimated, and the sequential events during deposition can be generally ascertained. Once a tract of contemporaneous depositional systems has been established and component facies identified, it is possible to predict potential traps and reservoirs within the seismic-stratigraphic unit.
The depositional mode interpreted for each successive seismic-stratigraphic unit provides the basis for inferring the overall tectonic and depositional evolution during basin fill and, in turn, points the stratigrapher toward potential prospects: types, stratigraphic position, geographic location and trend, structural situation, and reservoir character. At this point, some of the more sophisticated seismic analysis, involving velocity and amplitude variations, may be applied to prospective areas.
SEISMIC FACIES REFLECTIONS
A general description of the common seismic reflection patterns observed in Brazilian offshore basins will be described according to attitude, reflection configuration, continuity, amplitude, cycles, external geometry, associations, composition, and depositional mode. Seismic expression of a lithofacies has been called "seismic facies" by Sangree et al (1976); generally synonymous terms used here are "reflection unit" or "zone." An effort has been made to equate, where possible, differences in terminology for similar patterns.
Deltaic and Associated Systems
Probably the most important terrigenous clastic depositional system, in terms of volume of favorable reservoir facies, is the delta system (Figs. 1, 2, 3). A variety of Holocene and relict delta systems has been recognized and classified in several ways (LeBlanc, 1972; Fisher et al, 1969; Broussard, 1975). In general, a useful classification includes river-dominated, wave-dominated, and tidal-dominated deltas; naturally, many ancient and modern deltas developed in response to unique combinations of river and marine processes. Deltaic deposition was a principal mechanism for filling many sedimentary basins. Where combined with associated slope systems, the two systems account for most (by volume) of terrigenous clastic facies. Geologic studies have been focused on delta systems for the past two or three
decades. Fan-delta systems that are characteristic of the Brazilian post-rift or pull-apart basins have received very limited attention, although these types of deltas are common throughout the world (Fisher and Brown, 1972).
Two general types of delta systems have been recognized in the Brazilian offshore basins: delta systems tentatively inferred to be of the wave- and tide-dominated variety, and fan-delta systems.
A dip-oriented cross section and block diagrams illustrate the general arrangement of deltaic environments and associated lithofacies (Fig. 7A). The generalized cross section, which is uncomplicated by growth faults, schematically illustrates the common relative bedding attitudes within the delta system. The block diagrams demonstrate differences between deltas deposited primarily under the domination of river or marine processes. Specific differences involve composition and lateral continuity of river-dominated and marine-dominated delta facies, but the systems are similar in regional cross section and can be distinguished only by log patterns, cores, or large-scale, net-sandstone maps.
The seismic response to the two types of delta systems, along any dip line or profile, is similar, including growth faults, shale ridges, and inclined prodelta reflections. Thus, differentiation between an Eocene, river-dominated delta in the northern Gulf basin of Texas and a wave- and tide-dominated delta system like the Tertiary Niger system is not clear cut in seismic section, especially basinward of subjacent shelf edges where the delta progrades into deep ocean basins over associated slope facies. However, the Niger system is composed internally of barrier-bar, tidal-channel, and meanderbelt (point bar) sandstone facies (Weber, 1971), whereas the Gulf Coast Eocene deltas are composed of distributary-fill, channel-mouth bar, and delta-front sandstone facies (Fisher, 1969). Althou h reservoir characteristics will vary, both delta types commonly exhibit growth-fault and roll-over structural traps localized along delta depocenters (Fig. 7B). Updip pinchout of barrier sandstones is common in the wave-dominated system, and both delta types display downdip sandstone pinchout (See LeBlanc, 1972; Fisher and Brown, 1972; Broussard, 1975; among others, for specific references on delta systems).
Fan-delta systems are alluvial fans that prograde into marine or lacustrine environments (McGowen, 1970). These systems are coarse-grained, principally braided-stream deltas deposited under the influence of higher gradients and higher bed load (Fig. 8). They commonly are associated with fault basins where short, high-gradient streams flow from nearby source areas (Fig. 8B). The fan delta may be associated with tidal-flat or strandplain environments, depending on the nature and intensity of marine or lacustrine processes (Fig. 8B). A diagnostic feature of the fan delta is the common association with carbonate shelf facies. Internally, the fan delta, like the alluvial fan, is composed of proximal, medial, and distal clastic facies that exhibit a basinward decrease in grain size. Distal an facies are of transitional fan-marine origin (bars, barriers, tidal flats, or lagoons) and are developed by marine modification of fluvial facies (Fig. 8C). Prodelta facies are deposited from suspension beyond the distal sand deposits. Fan-delta facies may intertongue basinward and along strike with limestone and pelagic shale of the open-shelf environment.
Where faulting was contemporaneous with deposition, the fan-delta system thickens toward the fault and assumes a wedge-like geometry in longitudinal cross section (Fig. 8C). Brazilian fan deltas were probably wave and/or tidal-dominated (see Fisher and Brown, 1972, for references dealing with specific fan-delta processes and facies).
Seismic Facies Characteristics
Deltaic facies, recognized in offshore Brazil, can be grouped into several assemblages that exhibit reasonably diagnostic seismic reflection patterns: (1) prodelta and distal delta-front or barrier facies; (2) delta-front or barrier-bar facies; and (3) alluvial and delta-plain facies (Fig. 9A, B, C, F).
Prodelta and distal delta-front, barrier facies:
Reflection patterns for these facies in dip sections are horizontal to steeply inclined, oblique, layered patterns within a zone that ranges from poorly layered to reflection-free or locally chaotic. Oblique reflections may converge (and baselap) downward (basinward). In strike sections, the facies commonly exhibit convex-upward, conformable drape-to-mounded-chaotic, or reflection-free, patterns with some evidence of channel or gully erosion. Beyond the relict shelf edge (Fig. 9C, F), reflections are strongly divergent and inclined toward growth faults; reflections define rollover structures in dip section, and mounded, chaotic-to-conformable patterns in strike sections. On the relict shelf, the prodelta reflections are discontinuous except for a few strong reflections, amplitudes are ge erally low except for reflections with moderate continuity, and spacing is
Fig. 7. Delta depositional model illustrating the distribution of reservoir facies. A. General cross section of prograding delta system. Plan views of wave-dominated and river-dominated deltas demonstrate variations in lateral distribution of reservoir sand bodies. B. Schematic example of reservoir/trap conditions possible in deltaic systems. These systems commonly are involved in extensive growth faulting and shale diapirism.
very erratic. Beyond the relict shelf edge (Fig. 9C, F), reflection continuity increases, amplitudes increase, and spacing becomes more uniform. On the shelf, the external geometry of the reflection-bounded patterns define individual wedge to mounded units arrayed in offlapping, imbricate arrangement; collectively, the reflections constitute a tabular zone that gradually thickens basinward. Beyond the relict shelf edge in dip sections (Fig. 9C, F), the reflections compose a wedge that thickens against growth faults; in strike sections, the unit is characterized by convex-upward, lobate mounds with chaotic-to-conformable, lenticular patterns. The reflection patterns are terminated abruptly upward (toplap) by relatively horizontal delta-front or barrier-bar reflections (Fig. 9A, F). Bey nd the relict shelf edge, reflections are transitional downward into various types of slope reflections (Fig. 9C). The lithofacies that coincide with these reflections are massive units of laminated siltstone, mudstone, and some sandstone. The depositional mode of this reflection unit is inferred to represent, principally, suspension deposition on prodelta slopes with limited slumping and density flow--the unit is transitional between deposits of shallow-water deltaic, and/or barrier and deep-water slope environments. Depositional slope was approximately 1 to 5°.
Delta-front, barrier-bar facies:
Reflection patterns in dip sections that coincide with these facies are horizontal to slightly inclined, parallel-layered near the base, grading upward irregularly into chaotic or reflection-free patterns with common convex-upward diffractions and poorly defined, mounded reflections (Fig. 9A, F). Subtle, inclined reflections within chaotic zones may represent delta-front or barrier-bar offlap and, hence, may constitute internal time lines. In a strike section, the basal reflections of the zone exhibit drape patterns and local chaotic, to reflection-free, zones display subtle, parallel-layered to draped, reflections and abundant diffractions (Fig. 9B). Basal reflections exhibit strong continuity, but continuity diminishes upward in the unit. The best continuity occurs in dip sections. Amp itudes are moderate to high in basal, high-continuity reflections, but low in chaotic intervals; spacing is moderately uniform in basal reflectors, but erratic in the upper part of the zone. The reflections collectively define a tabular zone with minor thickening on the downthrown side of growth faults (Fig. 9C). The reflection unit is transitional with subjacent prodelta and distal delta front or
Fig. 8. General setting, facies, and facies associations that characterize fan-delta systems. A. Common setting in rift basins. B. Common association with carbonate facies and tidal flat-strandplain deposits. C. General cross section showing principal types of facies assemblages within the fan-delta system (after McGowen, unpublished).
Fig. 9. Deltaic and associated facies patterns generalized from reflection seismic sections showing characteristic reflection attitudes and continuity. Vertical scale in seconds (two-way travel time): A. Dip section of delta system. B. Strike section of delta system. C. Dip section of deltaic and subjacent slope facies; growth faults are commonly associated with these systems. D. Dip section of fan-delta system showing periodic marine-transgressive reflections. E. Dip section of coastal onlap by fan-delta facies over unconformity. F. Regional dip section of delta system that prograded across subjacent shelf-edge and slope facies. Principal growth faults occur basinward (right) where delta prograded into abyssal depths.
barrier reflections and is abruptly terminated upward by alluvial and delta plain reflections (Fig. 9A, F). Lithofacies within the zone are inferred to be extensive sandstone and interbedded shale sequences at the base, and superposed, probably lenticular, sandstone bodies in the upper part of the zone.
The depositional mode inferred for the reflection unit is shallow-water marine and delta front, or barrier bar with superimposed fluvial distributary-channel deposition. Shifting distributaries and repeated cycles of progradation and abandonment resulted in superposition of several delta or barrier sequences within the zone, especially on the downthrown sides of growth faults. Wave and, possibly, tidal action redeposited fluvial sands laterally and into deeper water. High-continuity reflections are inferred to represent marine-reworked facies. Chaotic to poorly defined reflection zones may be in response to thick, massive delta-front or barrier sandstone facies. Marine transgressive facies may be responsible for extensive, strong reflections at the top of the sequence in many areas. >
Alluvial, delta-plain facies:
Reflection patterns in dip sections that characterize these facies (Fig. 9A, C, F) are principally horizontal, parallel, rarely divergent, layered to locally reflection-free; locally, erosional channels maybe inferred. In strike sections, the reflections are weak, parallel-layered to subtle-mounded, chaotic-to-drape patterns. Continuity of reflections ranges from excellent to fair in dip sections (Fig. 9A, F), but continuity is poor to fair in strike sections (Fig. 9B); amplitude is variable (high in continuous reflections and poor in chaotic zones); and spacing is very regular in zones of high-continuity reflections but irregular in the remainder of the unit. The reflections collectively define a tabular external geometry, the base of which rises in the section in a basinward direction. The reflections overlie, and are transitional with, the delta-front or barrier-bar patterns and are overlain by chaotic, locally parallel-layered reflections with variable amplitudes and continuity that, probably, characterize the fluvial system that supplied the delta system.
The lithofacies resulting in these reflection patterns are massive sandstones and shales with inferred local channel-fill deposits. Thin marine shale and marl facies intertongue with the massive facies, especially in the lower part. The depositional mode inferred for this reflection unit is delta-plain and alluvial-plain processes involving tidal, distributary-channel, and meanderbelt deposition, within floodbasin and perhaps tidal-basin environments. Marine and delta destructional environments repeatedly transgressed the distal part of the delta plain.
Fan-delta facies in the Brazilian basins can be grouped into two assemblages that generally can be recognized from seismic data and verified by well information: (1) proximal- and medial-fan facies, and (2) distal-fan and prodelta facies. The intergradational facies can be separated only in a general, arbitrary manner by gradational changes basinward in seismic reflection configuration and continuity. These gradational changes are regional and can best be observed on regional dip sections. The following sections describe this basinal change in seismic characteristics (Figs. 9D, E; 10A, E, G).
Proximal, medial-fan facies:
Reflection patterns that develop in response to these facies are poorly defined, parallel-layered to reflection-free in both dip and strike sections (Fig. 10A); they may exhibit coastal onlap of erosional surfaces (Fig. 9E). Reflection continuity is absent to very poor in dip and strike sections, amplitudes are generally low, and spacing is relatively uniform. External geometry of the reflection unit is wedge-shaped, thickening toward the source area or toward bounding basement faults. The reflection unit thins basinward by losing section at the base by intertonguing with reflections of the distal fan-delta facies. The reflection unit is composed of massive conglomerate and coarse-grained sandstone, and some thin shale. It is inferred that the depositional mode was that of braided stream and channel-fill deposition in response to high gradients on the proximal and medial fan surface (Fig. 8). A nearby elevated source area has been postulated.
Distal fan-delta, prodelta facies:
This zone of reflections contains some poorly defined, inclined to horizontal, slightly divergent, layered reflectors alternating with reflection-free patterns. The number of reflections in the zone increases basinward and they may be gradational with well-developed shelf reflections (Fig. 10A, E, G). Some well-developed, inclined offlap reflections occur where the system progrades into deep water (Fig. 9D). The reflection continuity changes basinward from absent, to poor, to fair, and eventually grades into continuous shelf reflections (Fig. 10A). Amplitudes are variable and spacing is generally uniform. On stable shelf areas, the reflections collectively define a series of time-transgressive tabular units that overlap shelf carbonates basinward and are, in turn, overlapped by reflectio s of the proximal and medial fan-delta facies. In unstable basins (e.g., salt tectonic style) the reflection zone thickens basinward and may be subdivided into two or more units by thin, marine-transgressive
Fig. 10. Shelf and associated seismic facies patterns generalized from reflection seismic sections showing characteristic reflection attitudes and continuity. Vertical scale in seconds (two-way travel time): A. Dip section showing characteristic fan-delta/shelf transitional reflections. B. Dip section showing variations in reflection continuity and attitude that may represent reef or bank carbonate facies. C. Strike section of similar (B.) reflection variations. D. Inferred shelf-edge reef or bank outlined by termination of typical carbonate-shale reflections near shelf edge. E. Dip section showing periodic erosional surfaces in outer carbonate shelf; onlapping reflections terminate landward onto discontinuities. F. Strike section showing submarine canyon eroded into carbonate shelf a d filled by subsequent channel-fill deposits. G. Dip section of complex shelf-edge facies showing distinctive change in carbonate/shale reflections near the shelf edge; inferred to represent a reef complex with probable diagenetic alteration. H. Dip section of onlapping submarine canyon-fill deposits within a carbonate shelf system.
shale and marl that coincide with strong, widespread reflections punctuating the progradational cycles (Fig. 9D).
On shelf areas, these reflections grade landward and upward into poorly defined proximal and medial fan reflections, and basinward and downward into well-defined shelf patterns. Where the fans prograde into deeper water (Fig. 9E), the reflections grade basinward into various slope reflections. The lithofacies that give rise to the reflections are composed of marine sandstone and laminated siltstone and shale; sandstone facies are locally glauconitic. These were deposited by braided streams during floods, and were subsequently reworked and redeposited by wave and/or tidal processes to produce shallow-marine and possibly tidal-flat facies. Suspended sediment was carried basinward and deposited as a thin blanket of prodelta; some suspended sediment probably was transported along strike. n stable shelf areas, these distal facies intertongue with shelf limestone and shale deposits. Where the fan delta prograded to the shelf edge, the distal facies were redeposited in deep water by density currents and submarine slump processes.
Shelf and Associated Systems
One of the most characteristic elements of pull-apart basins (Klemme, 1971) is the shelf or platform carbonate system that is contemporaneously associated with coarse-grained delta systems and with mixed carbonate and terrigenous slope systems. Because of unique tectonic style, these carbonate shelf and platform facies were deposited in association with terrigenous clastic environments. The seismic reflection patterns that occur in response to the widespread sequences of limestone, pelagic shale, and marl are very distinctive (Fig. 10). The evenly bedded sequences produce reflection patterns that exhibit great continuity, high amplitudes, and relatively uniform spacing over thousands of square kilometers. Where these diagnostic reflection patterns vary, it is generally in response to ocal variations such as shelf-edge shoal, reef or bank, and submarine canyon-fill facies. Recognizing and mapping progressive positions of the carbonate shelf edges in these basins enable the geologist to determine the gross distribution of depositional systems.
Shelf systems in Brazilian offshore basins primarily are composed of: (1) widespread neritic limestone, marl, and pelagic shale, and localized reef, bank, or shelf-edge shoal facies that were deposited in slowly subsiding shelf environments that shifted basinward over prograding, principally terrigenous, clastic slope systems, supplied periodically by prograding fan-delta systems (Fig. 10); (2) shoal-water limestone, dolomite, and evaporite facies that were deposited in the absence of significant fan-delta deposition on relatively stable platforms, contemporaneous with limited deposition of mixed terrigenous and carbonate clastic and hemipelagic slope facies; and (3) thin, widespread, transgressive, biogenic, and terrigenous shelf clastic facies that were deposited during marine trans ression (coastal onlap) of abandoned deltas and fan deltas. The first two carbonate shelf and platform types constitute a large volume of strata that compose independent depositional systems. The latter shelf type, which is a minor component of the delta or fan-delta system, is discussed elsewhere in this report.
Reflection characteristics in the carbonate shelf and platform systems are considerably less variable than those exhibited by delta or slope systems, because of widespread, relatively uniform depositional environments. Furthermore, many variations in carbonate facies can be distinguished only by textural, petrographic, and fossil composition. Brazilian geologists have recognized a wide variety of carbonate facies, but few of these variations can be recognized using seismic reflection data. It has been possible, nevertheless, to recognize three principal facies assemblages based on well and seismic data: (1) widespread, moderate to low-energy, neritic, shelf limestone and shale facies; (2) local to moderately widespread, high-energy, shoal-water limestone facies composing reef, bank, p atform, and shelf-edge associations; and (3) submarine canyons filled with terrigenous and carbonate turbidites and pelagic facies.
Brazilian carbonate shelf systems resemble examples from the Upper Paleozoic of Texas in which deltaic clastic facies are restricted landward from carbonate-shelf and shelf-edge facies (Figs. 2, 3). However, Brazilian deltas associated with carbonate systems are principally of the fan-delta type. Brazilian platform carbonate systems, though, resemble Jurassic and Lower Cretaceous systems from the northern Gulf of Mexico (see Wilson, 1975, for references and analogous carbonate depositional systems).
Seismic Facies Characteristics
The shelf and platform facies of offshore Brazil are grouped into several assemblages that display distinctive seismic reflection patterns: (1) neritic shelf; (2) reef, bank, shoal-water, and shelf-edge facies; and (3) submarine canyon and channel fill (Figs. 9, 10, 15).
Seismic reflections that occur in response to these facies are generally horizontal,
parallel to slightly divergent or convergent, layered patterns. Reflection continuity is excellent, but pinchout occurs in convergent/divergent areas. Amplitudes of continuous reflections are high, and the reflections are uniform and closely spaced, except in local areas where reflectors diverge and converge. The reflections represent widespread, tabular units that are gradational updip with fan-delta patterns, and downdip with shelf-edge and various slope patterns. The patterns locally are terminated by submarine canyon erosion. The shelf sequence is composed of uniformly interbedded limestone, shale, and marl beds with great lateral continuity; thickness variations are slight and occur on a regional scale. These reflections are in response to facies that were deposited on broad, sta le shelf areas away from the influence of fan-delta deposition. The biogenic and pelagic facies define widespread neritic paleoenvironments.
Reef, bank, shoal, and shelf-edge facies:
Reflections in response to this group of facies are horizontal to steeply inclined, divergent to convergent, layered to reflection-free, or chaotic (Fig. 10B, C, D). Reflections exhibit poor continuity, variable amplitude, and irregular spacing. They compose a variety of lensoid to mounded units in dip sections; and in strike sections, the reflections generally compose an elongate unit that is parallel with the relict shelf edge. Reflections compose anomalous units within associated neritic shelf or platform reflections. Basinward, shelf-edge reflections may intertongue with subjacent slope reflections. Composition of the lithofacies responsible for these seismic reflections is massive limestone or dolomite, which may be highly altered by diagenetic processes. The massive carbonate facie are commonly composed of oolitic, algal, and calcarenitic limestone containing reef or bank fossils. The lithofacies are inferred to have been deposited in local, shallow-water reef, bank, and shoal environments subject to high wave and tidal energy. Anomalous reflections on middle and inner shelf areas probably represent lower energy facies than those on the relict shelf edges. Low energy lagoon facies also may be associated with the facies on the landward side of the seismic anomalies.
Submarine canyon-fill facies:
Reflections that characterize these facies (Fig. 10E, F, H) are horizontal to gently inclined, parallel to divergent and convergent, and layered to chaotic patterns that generally exhibit onlap (onlapping fill, Sangree et al, 1976). Reflection continuity varies from excellent to poor, amplitude is variable, and spacing is not uniform. Collectively, the reflections compose lensoid, canyon-fill units in strike section, and elongate, wedge-shaped units that thicken basinward in dip sections. Reflections occur within canyons cut into upper slope and shelf facies. The reflections may terminate by onlap against the walls of the canyon. Downdip, the reflections grade into onlap slope reflections.
Composition of the lithofacies varies from terrigenous to calcareous, clastic, and pelagic deposits. In map view, the reflection units may bifurcate updip and may be cut locally by growth faults. The reflections are inferred to represent submarine fan turbidites, slump deposits, hemipelagic, and, perhaps, neritic facies that were deposited in submarine canyons. Composition of canyon-fill deposits depends on the nature of the eroded shelf or fan-delta facies. Active fan deltas may have contributed sediment directly into the canyon during progradational episodes following canyon erosion.
Slope and Associated Systems
Beneath many continental shelf areas, and in many onshore basins, are slope systems that contain potential reservoir facies of variable quality and volume. Because these deep-water facies rarely crop out, except in highly complex orogenic areas, subsurface and seismic-stratigraphic methods are exceedingly important in their recognition and mapping. Considerable effort is being directed by marine geologists toward understanding modern slope processes and resulting sedimentary facies. By combining concepts of modern slope processes and facies with seismic geophysics, workers (such as Sangree et al, 1976) have begun to develop seismic-stratigraphic criteria for recognition of various deep-water facies. Seismic-stratigraphic methods provide an important tool for delineating, classifying, nd predicting slope facies composition, spatial arrangement, and stratigraphic relations. Because sparse well control is a severe limitation in deep offshore basins, seismic methods must be utilized extensively in stratigraphic, as well as structural, exploration. Slope exploration will improve with continued drilling, but at this time it is important for the explorationist to develop conceptual models that may permit better prediction of reservoir quality, arrangement, and trap potential.
Much has been published about turbidites and related facies, but effective exploration of slope systems will require much more information about slope processes, submarine fan composition, submarine canyon and channel characteristics, and the spatial arrangement of these facies. Integration of information obtained from modern
Fig. 11. General nature of seismic reflections that characterize several styles of deposition along some Holocene continental margins. A. Complex reflection patterns in deltaic and slope systems along southwestern African coast (after McMaster et al, 1970). B. Continental rise onlap along Nova Scotia coast (after Uchupi and Emery, 1967). C. Fault basins containing superposed (uplap) slope deposits dammed behind fault blocks, Baja California, Mexico (after Emery, 1970). D. Slope deposits trapped in salt basins and behind salt ridges along Louisiana coast, Gulf of Mexico (after Uchupi and Emery, 1968).
marine studies with results of conventional analysis of ancient slope systems is needed to support seismic-stratigraphic analysis of deep-water facies.
Conventional studies of deep-water reservoirs naturally have been focused on California basins for many years where the reservoirs have been so productive. Tertiary slope systems in Texas and Louisiana are being recognized (Fig. 1C), and Paleozoic intracontinental basins are being explored actively for deep-water reservoirs (Fig. 3A, C). It is natural that California basins were used as exploration models for deep-water reservoirs, but we suggest that the U.S. West Coast basins represent but one type of slope system, and that exploration of basins with different structural styles will require somewhat different concepts (see Middleton and Bouma, 1973; LeBlanc, 1972; Fisher and Brown, 1972; for references about slope systems).
Within the past 15 years, seismic cross sections of many continental margins have indicated the occurrence of various structural, stratigraphic, and sedimentary styles. Types of seismic reflections that we have observed in Cretaceous and Tertiary sequences in Brazilian offshore basins are herein called offlap/onlap, continental rise onlap, and fault-controlled and salt-controlled uplap; modern analogues are shown on Figure 11A, B, C, D, respectively. Recognition of these and other types of slope systems in ancient basins provides insight to the structural style and potential slope reservoirs that may occur. The distribution of facies within complex sequences of submarine fan deposits is also significant in evaluating slope reservoirs. Figure 12 illustrates Upper Paleozoic submarine fa facies, interpreted by using well control and by analogy with modern fans. The manner in which submarine fans may shift in response to subsidence, versus sediment supply, is an important factor in predicting the stratigraphic arrangement of ancient slope reservoirs.
Two fundamental slope relations that can be observed on seismic section may be called "offlap" and "onlap" (Fig. 13A). Onlap is common along modern slopes (Fig. 11B) and is inferred to be in response to erosion and redeposition of shelf and slope facies in the absence of a sustained supply of either shelf-edge or paralic sediments. Dietz (1963) presented an interpretation of continental rise onlap (Fig. 13B). In Brazilian offshore basins, we observed that extensive onlap deposition was commonly accompanied by some degree of canyon erosion of the adjacent shelf edge (Fig. 13A). We inferred that where the sediment supply diminishes and the shelf-edge retreats under long-term submarine erosion, slope-depositional environments gradually shift landward in response to the retreating and dim nishing
Fig. 12. Slope submarine fan model illustrating general processes and resulting composition that typify these deep-water depositional systems. Successive fans may offlap or may onlap, depending upon a sustained or diminishing sediment supply, respectively. Submarine fan deposits may stack in vertical or superposed manner if subsidence rates exceed sediment supply, thus producing an uplap system (after Galloway and Brown, 1973; based on Shepard et al, 1969; Carlson and Nelson, 1969; and Normark, 1970).
Fig. 13. The nature of offlap and onlap deposition--two fundamental depositional styles that characterize many slope systems. A. Block diagrams that illustrate general processes: offlap occurs during sustained sediment supply provided by deltas, fan deltas, and highly productive shelf-edge carbonate environments. Onlap conversely occurs when sediment supply diminishes and erosional processes rework shelf or paralic sediments, commonly via submarine canyons. Offlap reflections define the basinward progradation of slope deposits and onlap reflections mark periods of landward recession of slope depocenters. B. Schematic representation of onlap processes (after Dietz, 1963).
sediment source. Vail et al (this volume) referred to this type of reflection pattern as "marine onlap," and related canyon erosion and onlap to relative drop in sea level below the shelf edge (erosion), followed by a rise in relative sea level (onlap deposition). The progressive onlap of slope facies can be recognized in seismic sections.
Offlap seismic patterns in the Brazilian basins commonly coincide with periods of persistent deltaic progradation to the outer shelf, or to periods of excessive production of biogenic sediments along shelf-edge banks, reefs, and shoals. Therefore, offlap slope deposition is inferred to be in response to a sustained sediment supply that is greater than subsidence rates. During offlap, submarine fans and other slope facies shift progressively basinward as the basin is filled by slope deposition (Fig. 13A). Varieties of offlap (e.g., sigmoidal and oblique progradational, Sangree et al, 1976) probably result from variations in rates of progradation, versus subsidence. Onlap and offlap depositional models are illustrated on Figure 14 (I, II, III).
Another variation of slope sedimentation observed in Brazilian basins, as well as in seismic sections from elsewhere in the world, is herein called "uplap" (Fig. 14-I). Sangree et al (1976) called this slope relation "onlapping fill." It occurs in basins where subsidence rates are greater or equal to sediment supply, resulting in superposition of submarine fans and other slope deposits. Fault-controlled basins and salt grabens commonly exhibit this slope system, in which the deep-water facies onlap the flanks of the basin. Generalized examples of slope systems from Brazilian basins are shown in Figure 15; Figure 16 is a digrammatic representation of slope seismic reflections and inferred relations between slope type and the quality and distribution of potential reservoirs. We believe hat seismic discrimination presently provides the best basis for predicting slope reservoirs and traps in frontier basins.
Fig. 14. Examples of slope deposition in basins influenced by different tectonic styles and sediment supply. During development of a basin, each depositional type may occur. The onlap type may develop where sediment supply diminishes periodically. In rift and post-rift basins, there is commonly a progression from Type I to Type II and eventually to Type IV, with periodic episodes during which Type III may develop.
Fig. 15. Slope and associated seismic facies patterns generalized from reflection seismic sections. Heavier lines represent strong reflections and/or discontinuities. Vertical scale in seconds (two-way travel time): A. Dip section of a slope complex composed of older onlap rise facies and younger offlap/onlap slope facies. Onlap reflections terminate along strong local or regional reflections and/or discontinuities in reflection attitudes. B. Dip section through slope composed of chaotic reflections enveloped in sigmoid-shape offlap units. Complex offlap/onlap reflections shown in section A. may represent discrete, alternating episodes of offlap deposition followed by episodes of shelf-edge, slope erosion and extensive onlap. The sigmoid-shaped units composed of onlap and chaotic refl ctions may, on the other hand, reflect rapid offlap, oversteepening of slopes, extensive slumping, and local onlap of submarine fan and proximal slope deposits. C. Dip section of slope system showing relatively uniform continuity of offlap reflections (commonly calcareous/terrigeneous, hemipelagic, upper slope facies) representing relatively slow rates of deposition. D. Dip section of slope system showing relatively uniform, but truncated (sometimes called "oblique"), offlap reflections indicating relatively rapid progradation and minimum subsidence. E. Dip section through thick onlap slope units representing extensive continental rise deposition; internal reflections terminate along strong reflection discontinuities. An offlap/onlap complex overlies the rise deposits representing anothe depositional episode. F. Strike section of offlap/onlap slope system characterized by convex-upward reflectors representing hemipelagic blankets draping large slope fan complexes (or cones). G. Dip section through a slope system characterized by uplap reflections caused by rapid subsidence of salt basins. H. Dip section through a slope system composed of uplap reflections resulting from subsidence of minor fault block.
Fig. 16. Seismic-stratigraphic slope facies patterns and inferred distribution of submarine fan reservoirs. A. Schematic representation of reflection patterns that characterize offlap, onlap, and uplap slope facies. B. Inferred distribution of submarine fan sandstone facies in three principal types of slope systems.
Seismic Facies Characteristics
Seismic reflection patterns that characterize offlap, onlap, and uplap slope relations will be described and related to inferred lithofacies. Mixed offlap and onlap variations will be described separately (Fig. 15A, B).
Offlap slope facies:
Both uniform offlap and truncated offlap reflections have been recognized (Figs. 15C, D; 16A). These patterns correspond, respectively, to sigmoid terrace and oblique progradational seismic facies (Sangree, et al, 1976). The reflections are inclined, parallel to divergent to convergent, layered with the dip of reflections decreasing near the base of the zone. In strike sections, the reflections commonly exhibit mounded and draped patterns. Continuity of reflections is high in uniform offlap and moderate in truncated offlap. Amplitude is high in uniform offlap reflections and high to moderate in truncated off lap reflections. Spacing is generally uniform in offlap reflections but more irregular in truncated offlap reflections. The reflections collectively compose a wedge to lensoid-shaped unit that pinches out updip and downdip (Fig. 16A); individual reflection units are inclined, generally recurved lenses or wedges that converge and pinch out basinward (baselap) and are terminated updip (toplap) by the basal shelf surface (Fig. 15C, D). In strike sections, the individual offlap reflection units are characterized by convex-upward, mounded reflections. The lithofacies that coincide with these reflections are hemipelagic shale and/or calcilutite, lenticular turbidite sandstone or calcarenite, and slump deposits. Offlap facies are inferred to have been deposited by turbidite deposition on submarine fans, by slumping, and by hemipelagic sedimentation over the entire slope surface. Sustained sediment supply was greater than subsidence, resulting in the offlap pattern of deposi ion. Calcareous slope deposits are common in the uniform offlap type of system; terrigenous sandstone and shale facies are common in truncated offlap reflection units.
Onlap slope facies:
Onlap reflection patterns define thick, continental rise deposits (Figs. 15E; 16A), as well as elements within alternating offlap and onlap slope patterns (Fig. 15A, B). Within alternating offlap and onlap units, the onlap reflections may be well defined (Figs. 15A; 16A), or poorly defined to chaotic (Figs. 15B; 16A).
These reflections are slightly inclined to horizontal, onlapping, parallel-layered to chaotic patterns in dip sections. In strike sections, the reflections define horizontal to draped, parallel-layered to chaotic patterns. Reflection continuity is fair to excellent in thick, continental rise units, but fair to poor in onlap units that alternate with offlap reflections (Figs. 15A, B; 16A). Amplitude is high to moderate, except in chaotic units, and spacing is relatively uniform in continental-rise facies, but less uniform in other onlap types. The reflections collectively compose a wedge to lensoid-shaped sequence of strata that pinches out or is truncated updip by shelf reflections (toplap), and pinches out downdip (baselap). In strike sections, the onlap unit exhibits a draped or cha tic mound geometry. Lithic composition is similar to offlap facies, although onlap lithofacies may be coarser grained if erosion has cut into fan-delta facies preserved on the relict shelf. Onlap facies are inferred to represent deposition under conditions of diminishing sediment supply. Vail et al (this volume) cited that this type of onlap, marine onlap, developed during sea-level rise when the continental shelf was subaerially exposed and rivers deposited sediment directly into deep water through eroded shelf-edge canyons. We inferred that sediment supply was derived primarily by submarine erosion of shelf, slope, or fan-delta deposits by submarine canyons. Some littoral sediments may have entered canyons if they extended into the longshore zones.
Continental rise units represent long-term episodes of onlap (Fig. 16A); alternating offlap and onlap reflections indicate shorter-lived periods of onlap, perhaps mixed with brief episodes of rapid, localized offlap. Chaotic onlap is common where slopes were steeper and submarine slumping was common; more regular onlap reflections occur on slopes or within canyons with lesser gradients. Submarine fan reservoirs in onlap facies are inferred to possess better potential than in offlap systems, since erosion may tap coarse, deltaic facies, and the onlap process provides for better updip pinchout possibilities (Fig. 16B).
Mixed offlap and onlap facies:
These systems are common in Brazilian basins where the two slope-depositional modes commonly alternated during a prolonged progradational episode (Fig. 15A, B, F). Offlap deposition apparently terminated periodically, followed by extensive, perhaps local, submarine erosion of shelf edges and slope to supply onlapping slope environments. Renewed fan-delta deposition subsequently reactivated slope offlap or progradation. Although large volumes of slope facies are of the onlap variety, the onlap units are arranged within sigmoid-shaped offlap wedges (Fig. 15B). Widespread high-amplitude reflections can be traced from the shelf, through the shelf-edge facies, and downward through the slope system. Onlapping slope reflections terminate updip along these discontinuities indicating that a long eriod of slope
onlap coincided with shelf erosion (Fig. 15A, B). Seismic reflections within the offlap and onlap sequences are similar to those within independent systems previously described.
Uplap slope facies:
To sustain slope deposition to produce a superposed arrangement of seismic reflections requires that subsidence and sediment supply are relatively balanced. Superposed slope-turbidite and hemipelagic facies of this type are common in fault basins and salt grabens that underwent subsidence during deposition (Figs. 15H; 16A). The reflections exhibit onlap against the steep flanks of the basins and may also display a variety of drape reflections over paleobathymetric highs.
The reflections are horizontal to slightly inclined, parallel-layered to rarely chaotic, or reflection-free patterns. Excellent continuity may be displayed, or continuity may be poor to missing in chaotic or reflector-free zones. Amplitude is high in zones with reflector continuity, and spacing is uniform within the layered, probably hemipelagic zones. The reflections compose irregular, concave-upward, lensoid to wedge-shaped units that may exhibit local drape structure (Fig. 16A). Composition of the facies is similar to offlap slope facies, but may be higher in coarse clastic sediment if the adjacent shelf is narrow and subjected to repeated fan-delta deposition (Fig. 14-I). Lithofacies exhibiting these types of reflections were deposited in subsiding, deep-water basins associated wi h faulting or salt tectonics. The basins were filled primarily during periods of sustained sediment supply, but deposition may evolve into an onlap phase. Because such basins commonly exhibit bathymetric relief, it is inferred that coarser, density-flow deposits will be concentrated within the paleobathymetric depressions, which may not coincide with structural highs (Fig. 16B).
SEISMIC-STRATIGRAPHIC MODELS OF OFFSHORE BRAZILIAN BASINS
Integration of the various delta, shelf, and slope depositional systems, characterized by facies that generally can be recognized by distinctive seismic reflection patterns, provided the basis for construction of a series of Brazilian basin models (Figs. 17-21). These models are generally typical of those that developed in early rift and subsequent pull-apart areas of the world, and are based principally on seismic criteria tied to available well control. They show the general variety and interrelation of depositional systems that developed in response to various tectonic styles since rifting was initiated. With variations, each basin developed through two or more of these types during its post-rift history. The models are intended to illustrate an integration of well data and seismic stratigraphic interpretation.
Early Rift-Basin Model
Basins that formed during early stages of rifting (Figs. 17; 18A), and which were isolated sufficiently from the proto-Atlantic, are commonly characterized by facies that indicate initial lacustrine environments that changed gradually to marine conditions. Subsequent marine environments were either normal or hypersaline. These basins were commonly sediment-starved initially, but an extensive sediment supply generally developed from nearby upthrown fault blocks; faulting was commonly contemporaneous with sedimentation.
Depositional systems tract:
and Alluvial fan, fan delta, periodic transgressive shelf, and offlap-uplap slope systems compose the typical sequence or tract of depositional systems from source to deep basin.
The basins are bounded by faults and commonly tilted toward the continent. Faulting was contemporaneous with deposition, but slowly diminished in intensity. Shale diapirs developed in the basin in response to excessive loading of fan-delta sands on thick prodelta and slope facies; and shale movement continued until late in the history of the basin.
Principal depositional modes:
Adjacent elevated source areas supplied large volumes of coarse-grained clastic sediment to the basin along short braided streams that crossed boundary faults to fan-delta depocenters. An integration of drainage supplied fan deltas that prograded along the axis of the basin from more distant sources. Rapid deposition and subsidence (including shale diapirism) precluded significant carbonate deposition, but thin biogenic shelf facies transgressed the fan-delta surface periodically in response to delta shifting, or minor eustatic or tectonic sea-level changes. Slope facies are principally of the offlap variety or (in cases of rapid basin subsidence) the uplap type. Submarine fan deposits were derived from slumping distal fan-delta, sand facies.
Source area and drainage basin:
Sediment was derived principally from Paleozoic strata or basement rocks landward of the rift zone. The drainage system was poorly organized and originated in nearby elevated areas beyond the boundary faults.
Post-Rift Salt-Basin Model
Basins that formed during early stages of rifting and that were connected with the proto-Atlantic, via restricted openings, were commonly the site of extensive salt deposition (Figs. 17;
18B). As continental drift continued, normal marine circulation developed, and the basins became the sites of extensive clastic deposition when adjacent fault-bounded source areas were periodically elevated. Mobilization of the subjacent, bedded salt generated a relatively unique tectonic style and imposed a distinctive depositional mode on these basins. Rarely, in the absence of a clastic source, limestone was deposited directly on the bedded salt deposits, resulting in carbonate platform systems that exhibit down-to-the-basin growth faults.
Depositional systems tract:
Alluvial fan, fan-delta, localized shelf-carbonate, and uplap slope systems typify the basinward sequence or tract of depositional systems in a post-rift salt basin. In areas where salt thickness diminishes, more normal offlap and onlap slope systems dominate.
These basins are bounded on the continental margin by extensive, regional fault zones that separate the basin from adjacent source areas. The reactivated basement faults originated during early rift stages of basin development. Faulting may have been contemporaneous with deposition, but more commonly, periodic fault activation occurred, accompanied by erosion of fan-delta facies along the basin margin, and followed by coastal onlap of fan-delta facies over the erosion surface. Salt diapirism was a dominant element in the tectonic history of these basins, when salt, mobilized by prograding fan-delta systems, moved basinward into salt ridges, and locally upward to form diapirs. Salt tectonic instability precluded development of well-defined carbonate shelf edges. Uplap slope facies were de osited in subsiding salt grabens and basins.
Principal depositional modes:
Elevated source areas along the continental margin supplied fan-delta systems with coarse-clastic sediment. Periodic faulting rejuvenated the source areas. Fandeltas prograded basinward, locally over extensive tidal-flat environments, but were trapped landward of large salt ridges. Periodically, biogenic
Fig. 17. General depositional models of a rift basin and a salt basin showing schematic representation of reflection attitudes and continuity (see Figures 9, 10, 15 for additional detail of reflection attitudes and continuity). The rift-basin example is characterized by rapid deposition of fan-delta and slope facies in contemporaneously faulted basins; slope systems are commonly represented by offlap and uplap reflections in response to rapid progradation and rapid subsidence. Similarly, fan-delta progradation across thick salt deposits produced salt mobilization resulting in salt dams and subsiding salt basins containing uplap facies.
Fig. 18. Dip sections across margins of three pull-apart basins showing general distribution of depositional systems delineated and mapped, in part, using seismic-stratigraphic criteria. A. Basin showing transition from rift-basin deposition to passive, ocean basin deposition. B. Basin filled under the influence of salt tectonics and adjacent elevated source area. C. Basin that evolved from rift type to passive offlap type to integrated deltaic/slope type. Horizontal scale: schematic. Roman numerals signify principal seismic-stratigraphic units delineated by regional reflections.
clastic and carbonate shelf facies trangressed the fan deltas; local platform carbonate (reef or bank) facies formed on positive structural elements. Distal fan-delta sediments provided the source of coarse-grained turbidite fan facies that were deposited in an uplap attitude within the subsiding salt basins and grabens. In salt basins without a clastic source, thick carbonate facies developed on top of the salt, generating local growth faults in the carbonate sequence.
Source area and drainage basin:
Sediment was derived from elevated Paleozoic and basement rocks adjacent to the boundary faults. The drainage system was a short and poorly developed braided complex. Fan-delta systems exhibit extensive red-bed facies that developed on the fan surface during the course of basin development. The region may have evolved from an arid climate to a warm, humid climate with high rainfall, as a result of the growing size of the Atlantic Ocean.
Post-Rift Carbonate Platform Model
After initial rifting, basins with limited sediment supply were the sites of extensive aggradation or upbuilding by platform carbonate deposition (Figs. 19; 18A). Although limited terrigenous sediment reached the basin through fan deltas, it was generally trapped landward in rift basins that were undergoing final subsidence. The carbonate platform systems exhibit limited progradation, principally from deposition of hemipelagic carbonate deposits and carbonate turbidites. The platforms shifted basinward to produce a series of superposed, slightly offlapping shelf edges. Periodically, the platforms were subjected to inferred episodes of submarine canyon erosion. During platform carbonate deposition, the basin was essentially sediment-starved, except for in-situ biogenic sediment.
Depositional systems tract:
Fan delta, carbonate platform, shelf edge, calcareous and offlap slope systems comprised the environmental tract at that time in basin history. Fan-delta systems were restricted near source areas, and the broad platform was the site of shallow-water carbonate deposition. Slopes were principally of the offlap variety, but some onlap sequences developed during periods of inferred submarine canyon erosion and deposition, especially terminal stages.
Fig. 19. General depositional model of carbonate platform complex in a post-rift pull-apart basin showing schematic representation of reflection attitudes and continuity (see Figures 9, 10, 15 for additional detail of reflection signatures). This early post-rift basin is characterized by offlaping shelf-slope systems of predominantly carbonate composition. I. Landward, minor fan-delta systems periodically supplied terrigenous sediment to the platform, but principal shelf/slope facies are limestone deposits. II. Shelf-edge reefs are common and the shelf edge prograded by alternating deposition of calcareous offlap slope facies and onlap slope facies commonly associated with submarine canyon systems.
Although bounded on the landward margin by extensive basement faults, the basin experienced slow, relatively uniform subsidence, resulting in deposition of thick limestone facies on the shallow platform. Where the carbonate platform sequence was underlain by salt or mobile, highly pressured shale, some tectonic instability was possible. Growth faulting associated with shale or salt diapirs (or ridges) resulted in deposition of thick sections adjacent to the contemporaneous faults. Conversely, movement on boundary basement faults resulted in the deposition of thin, commonly shoal-water facies on upthrown fault blocks; deeper water facies accumulated in downthrown fault blocks.
Principal depositional modes:
In response to limited terrigenous clastic sediment supply and relatively uniform subsidence rates, the basin (Fig. 19) was the site of a variety of constructive carbonate facies tracts, including reef or bank, lagoon, open-shelf, and shelf-edge complexes. Algal facies denote commonly shallow-water environments, although neritic environments were common. Evaporite deposition also occurred in proximal areas. Carbonate facies intertongue updip with fan-delta or tidal clastic deposits. Downdip, the platform facies grade into steeply dipping calcareous or terrigenous clastic slope deposits. Slope facies commonly display relatively uniform offlap in which considerable reflection continuity exists from shelf to slope. Depositional rates were probably slow since aggradational platform depositio al units commonly can be traced over the shelf edge and into the slope sequence. Calcarenites that are common on shelf edges and upper slopes grade basinward into calcilutites; however, calcarenite submarine fan deposition occurred locally on the lower slope. During periodic episodes (Fig. 19), inferred submarine canyon erosion developed along shelf edges and locally extended landward into the platform and shelf environment. The final canyon erosional episode commonly produced thick, onlapping continental rise deposits that lapped far up the eroded slope. Sediments eroded from carbonate shelf edges were transported by density flow to onlapping submarine fans. Hemipelagic carbonate sediment also settled on the slope, especially on upper and middle slope surfaces. Submarine fans onlapped t e slope and, at least partially, filled submarine canyons. Pelagic carbonates also were deposited in the canyon systems.
Source area and drainage basin:
Source areas were either low and unimportant or sediment was being effectively trapped within intermediate fault basins. Drainage was restricted or, perhaps, diverted into other adjacent basins.
Passive Offlap Model
When basement subsidence, rift faulting, and structural activity associated with salt and/or shale mobilization diminished in the pull-apart basins, the depositional mode slowly shifted from rift-related to passive-basin sedimentation (Figs. 20; 18A, B). When rift-related basement faulting terminated, depocenters shifted into marginal marine basins in the growing Atlantic Ocean. The depositional style or mode was dominated by extensive offlap of slope depositional systems onto deep oceanic crust. A relatively continuous sediment supply maintained these progradational episodes for long periods of time, interrupted only by periodic, perhaps lengthy, episodes of shelf-edge and slope erosion, and local submarine canyon development. Strata deposited in this style of basin comprise much of he sedimentary volume preserved along Brazilian coastlines, where an extensive, integrated drainage system was absent. Elevated continental marginal areas were the principal source areas.
Depositional systems tract:
Fan-delta, carbonate-shelf, shelf-edge, and slope systems comprise the environmental tract that was responsible for depositing large volumes of carbonate and clastic facies. Slope systems exhibit alternating offlap and onlap varieties of deposition. Onlap slope deposition tended to dominate, especially in the older, landward slope systems, but younger slope facies commonly display better developed offlap (sigmoid type) of a more calacreous composition. Beneath many of these slope systems occur thick, onlapping continental rise sequences deposited in response to a long period of submarine erosion of older carbonate platform systems (see post-rift, carbonate platform model).
Depositional offlap into marginal ocean basins occurred with minimum structural control, other than regional subsidence. Compaction of thick clastic sequences also contributed significantly to regional subsidence. Source areas were maintained by relatively continuous uplift. In some basin, growth faulting developed progressively at the shelf edge-slope break, where thick sequences of undercompacted muds failed on steep slopes.
Principal depositional modes:
Fan-delta systems repeatedly prograded across carbonate shelf environments, thus intertonguing the coarse deltaic clastics with open-shelf carbonate facies. Reworked deltaic sands and muds were transported by longshore currents along strike and, perhaps, basinward by tidal currents. Abandoned fan-delta lobes were periodically transgressed by open-marine carbonate environments. Carbonate facies
Fig. 20. General depositional model of fan-delta, carbonate-shelf, and slope systems in a passive pull-apart basin showing schematic representation of reflection attitudes and continuity (see Figures 9, 10, 15 for additional detail of reflection signatures). This depositional setting was responsible for depositing a significant part of the fill in these basins. Active fan-deltas supplied terrigenous clastic sediment to the shelf area where limestone deposition dominated; this sustained episode of clastic/carbonate sediment supply produced a steady progradation of offlap slope facies I. Alternating with episodes of sustained progradation of shelf/slope environments were episodes of diminished sediment supply with corresponding erosion of shelf edges and onlap of calcareous/clastic slop facies II. The terrigenous clastic influence generally diminished through time, and slope systems became increasingly calcareous in composition.
were deposited in open-marine neritic and shelf-edge bank or reef environments. Slope systems exhibit complex offlap and onlap seismic patterns (generally oblique progradational) dominated by poorly defined onlap reflections (mounded chaotic) that commonly extend upslope into submarine canyons. The alternation of offlap and onlap facies indicates that the slope system was constructed by offlap of submarine fans, maintained by sustained terrigenous and carbonate sediment supply, and followed, perhaps, by long periods of erosion, slumping, and onlap of submarine fans (Fig. 20). Terrigenous clastic sediment supply slowly diminished during this depositional mode and an increasingly calcareous, dominantly offlapping slope (sigmoid progradational) characterizes the younger deposits.
Source area and drainage basin:
Source areas were located along the basin margin as a series of elevated coastal ranges. Drainage systems were braided, and discharge was probably continuous due to higher rainfall along the coastal ranges of the relatively broad south Altantic Ocean.
Late Tertiary Delta Model
Eventual integration of rivers into systems that drained large continental interior areas, focused immense volumes of sediment into a few large deltaic depocenters. In Brazil, this type of major oceanic delta differed from the fan-delta systems common in most post-rift, pull-apart basins. Discharge was higher, and bedload to suspended load ratios were much lower in the delta system than in fan-delta systems. The Brazilian delta prograded rapidly across a continental shelf and initiated shelf-margin deltaic sedimentation (Figs. 21, 18C). Redeposited deltaic sediments were transported into deep water by slumping and by turbidity flow, resulting in deposition of thick, generally offlapping slope facies over which the shelf-margin deltas prograded. Growth faulting and shale diapirism, whi h were common adjuncts
Fig. 21. General depositional model of delta/slope systems in Tertiary pull-apart ocean basins showing schematic representation of reflection attitudes and continuity (see Figures 9, 10, 15 for additional detail of reflector signatures). This style of deposition reflects development of an integrated drainage system in the pull-apart basin, resulting in deposition of thick deltaic and slope facies. Prograding deltas built across subjacent carbonate platforms I. to shelf-edge positions where they supplied sediment that was redeposited by slope processes into deep-water environments. Steep slopes generated by sustained deltaic deposition at the shelf edge resulted in development of growth faults and shale diapirs II. Delta systems may be either marine or river dominated; slope systems ar principally of the offlap type.
to deltaic and slope deposition, developed where water-saturated prodelta and slope muds failed under the sedimentary load. Wave and tidal processes are inferred to have dominated late Tertiary Brazilian delta facies.
Depositional systems tract:
Fluvial, delta, and slope systems characterize the tract within this type of major oceanic depocenter. The systems prograded over submerged shelf systems until reaching the relict shelf edge, where deposition was directly into the deep oceanic basin. Minor and localized episodes of marine transgression occurred where deltaic lobes were temporarily abandoned by river shifting.
Distant source areas were probably affected by late Cenozoic tectonism in the Andes region. In the ocean basin, structural activity involved principally regional subsidence and sedimentary tectonics such as growth faulting and shale diapirism. The nature and spatial arrangement of deltaic and slope facies were significantly controlled by growth faulting. Slope and deltaic facies thicken dramatically toward contemporaneous faults. Slope shale, displaced by the fault blocks, migrated basinward into elongate shale ridges parallel with the basin margin.
Principal depositional modes:
The delta system prograded basinward as a series of offlapping, imbricate wedges of sediment, deposited by shifting depocenters. Well-log patterns indicate that the delta systems were of wave- and tide-dominated variety, although evidence is not conclusive. The prodelta facies exhibits moderately well-developed offlap seismic reflections (oblique progradational) that are inclined toward the basin, beneath relatively horizontal reflections that represent delta-front or barrier and delta-plain facies (Fig. 21). Progradation rates exceeded subsidence rates on the continental shelf, but progradation probably diminished when the delta system reached deeper water beyond the shelf edge. Deltas remained relatively stationary near growth faults until displacement diminished, at which time they pr graded basinward until another growth fault developed. Slope sedimentation was concentrated in front of the prograding delta where prodelta and delta front deposits were reworked and transported into deep water by density processes. Growth faulting and diapirism have significantly modified the primary attitude of deltaic and slope depositional surfaces. Marine environments locally transgressed inactive delta lobes, resulting in deposition of marl, shale, and glauconitic sandstone.
Source area and drainage basin:
Source areas were within the continental interior, and the drainage system was an integrated complex that maintained relatively high discharge most of the year. The river transported dominantly suspended sediment.
The combined use of seismic reflection data and current basin-analysis concepts provides a potent exploration tool in basins where well control is limited. Even where well control is dense, the use of seismic-stratigraphic interpretations will permit maximum use of well data. Conventional facies interpretations, based on well data, can be tested and extrapolated throughout a basin using a time-stratigraphic framework constructed using regional seismic reflections that conform to depositional surfaces. The external geometry and distribution of zones that reflect distinctive configuration and continuity (seismic reflection units) can be calibrated with available well data to provide insight to the nature of the strata that fill the basin. Even where seismic reflections cannot be tied to well data, the character of the reflection unit and its spatial association with other groups of reflections may permit uncommonly accurate prediction of lithofacies composition and, hence, facies patterns within the basin. This prediction capability, permitted by use of seismic data in stratigraphic interpretation, can lead to logical interpretations of petroleum potential long before well control provides sufficient data for conventional stratigraphic interpretation.
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(2) University of Texas at Austin, University Station, Box X, Austin, Texas 78712.
Many Petrobras geologists and geophysicists were involved in the offshore basin analyses. These explorationists provided expertise in both geology and geophysics which contributed significantly to the program. The professional contribution and personal support of these associates are gratefully acknowledged: Hildeberto Ojeda y Ojeda, Ercilio Gama, Jr., C. S. Baumgarten, J. A. Estrela Braga, Roberto Morales, Henrique Della Piazza, G. Estrella, J. B. Gomes, M. V. Dauzacker, M. Saito, A. M. Fugita, and K. Tsubone. Special thanks are extended to Carlos Walter Campos, Chief of Exploration; Renato Pontes, Chief of Geology; Juarez Tessis, Chief of Geophysics; and Wagner Feriere, Subchief of the Risk Contract Program and formerly Chief of Geophysics. Permission to use illustrations and genera ized information from the Brazilian basins for this report was granted by the Directorate of Petrobras.
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