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AAPG Bulletin

Abstract


Volume: 60 (1976)

Issue: 3. (March)

First Page: 379

Last Page: 388

Title: Complex Fault Structures in Veracruz Province of Mexico

Author(s): R. W. Mossman (2), Francisco Viniegra (3)

Abstract:

Tertiary rocks of the Veracruz basin overlap westward onto overthrust Cretaceous limestones and infolded lower Tertiary clastic rocks, which comprise the eastern flank of the Sierra Madre Oriental. Evaluation of the structural relations of the buried foothills was accomplished through close integration of the extensive seismic and borehole information available. This involved deciphering details of the complex fault system by study of the vast number of diffractions associated with the seismic data, as well as the resolution of the occasionally ambiguous relations of the well data. A middle Eocene age is established for the orogeny. The concepts derived from this investigation have significant tectonic implications as well as potential in guiding economic development.

/CJSABSTRACT>

Text:

INTRODUCTION

The Veracruz province of Mexico encompasses approximately the south-central part of the state of Veracruz (Fig. 1). It is separated from the old productive area of Poza Rica on the north by a zone of Tertiary volcanism consisting of the Santa Ana-Teziutlan massif. On the southeast there is a less well-defined separation between the Veracruz province and the Reforma area, where recent petroleum development has occurred in the states of Chiapas and Tabasco. Such demarcation, however, appears to be approximately coincident with the San Andres massif, also a zone of Tertiary volcanic rocks.

The province is often referred to as the Veracruz basin although the basinal part actually consists only of the area on the east containing thick Tertiary sedimentary rocks. The Tertiary basin is characterized by north-south-trending anticlines, bounded on the seaward side by extensive, down-to-the-east growth faults. On the west (Fig. 2), the Tertiary beds overlap the buried foothills of the Sierra Madre Oriental which are productive locally of oil and gas. These complex, buried structures thus are of particular economic interest. They comprise extensively overthrust, imbricate plates of Cretaceous limestones with associated, locally infolded lower Eocene and Paleocene shales.

About 2,000 km of seismic profiles, on approximately a 2-km grid, provides detailed seismic coverage of an area of more than 4,000 sq km within the limestone zone. The seismic data were recorded using either digital or analog magnetic-recording methods, and most such recording involved common-depth-point techniques. Figure 3 shows the extent of the seismic grid in the foothills zone.

In addition to the seismic control, nearly 100 wells have been drilled in the area, about one-half of which represent exploitation drilling, principally in Angostura field. Most of the wells were drilled after 1950, and the available data include standard electric logs, gamma ray-neutron logs, acoustic logs, detailed sample logs, lithologic and mineral analyses, paleontologic analyses of the microfauna, and core analyses. The locations of most of these tests also are shown on Figure 3, although some wells are omitted where several are in close juxtaposition.

It is difficult to determine structural and chronological relations in an area of complex faulting, and hence to assay any economic potential that may exist. Characteristically, in the Veracruz province the seismic data are replete with diffractions and usually are obscure; also the relations of the wells not always are defined clearly. However, the data have been integrated into positive geologic concepts which are pertinent to the regional tectonics of the area and provide guidance for commercial exploitation.

STRATIGRAPHY

The stratigraphic relations within the province are shown by the columnar section in Figure 4. The Miocene to recent strata consist primarily of shales with locally interbedded fine sandstones, whereas coarse clastic rocks are present locally in the basal Miocene, the Oligocene, and the upper Eocene. The middle and lower Eocene and Paleocene consist principally of shales and sandy shales, with coarse clastic rocks rarely present.

In the western part of the Veracruz province, that part which composes the foothills of the Sierra Madre Oriental, an extensive sequence of Upper and middle Cretaceous limestones and shaly limestones is present beneath the Tertiary. The true sedimentary thickness of these limestones is

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Fig. 1. Map of eastern Mexico with inset of Veracruz province.

Fig. 2. Schematic cross section across Veracruz province.

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Fig. 3. Location of seismic lines, oil and gas production, and principal dry holes in zone of foothills structure.

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not established because of the complex, imbricate structural conditions. Nor are rocks of Early Cretaceous age recognized in the buried foothills, except about 20 km northwest of Fortin in the Sierra Madre where Lower Cretaceous limestones equivalent to the lower Tamaulipas are present in a limited outcrop area.

Jurassic red beds are believed to underlie the Cretaceous in the foothills belt, but this postulation is based primarily on extrapolation of relations from areas on the west.

Of particular interest within the stratigraphic sequence in the area of foothills structures is the middle Eocene unconformity. This hiatus is thought to be primarily the result of structural disturbance and lack of deposition rather than extensive erosion. This is demonstrated readily in the deeper, eastern parts of the overthrust-fault belt, but is defined less well on the west where subsequent erosion has occurred. East of the eastern limits of the zone of faulting a completely conformable middle Eocene section, consisting of shales of the Guayabal Formation, is present.

The first stratigraphic unit beneath the middle Eocene unconformity differs from one location to another as a function of structural conditions. In extreme cases upper Eocene Chapopote shales, which overlie the unconformity in most of the area, are in contact with inverted middle Cretaceous limestones. The stratigraphic sequence (Fig. 4) represents the normal vertical continuity of sediments, undistorted by structural deformation.

STRUCTURE

The presence of overthrust faulting has been recognized for at least 25 years in this area, since before the first oil production in the province was discovered in 1953 at Angostura, but only recently has the full intensity of the overthrusting been realized. Originally one or two, no more than four, thrust plates were postulated, but present studies have identified as many as 18 sequential imbricate plates in parts of the subsurface area, with additional thrust faulting farther west where outcrops of the rocks are present.

The zone of overthrusting was mapped by seismic means for a linear distance greater than 130 km along the strike of the faults, which is generally north-northwest. A part of this zone of faulting near Angostura field is shown in Figure 5. The lines on the subcrop map are fault traces, not contours, and mark the intersection of the thrust faults with the northeastward-sloping unconformity surface. The distance between the faults as they intersect the unconformity ranges from ½ to 3 or 4 km, and they appear to coalesce at depth.

Fig. 4. Columnar section, Veracruz province.

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The deepest and most eastern of the fault blocks is more than 3,500 m in depth at its uppermost limit against the unconformity.

AGE OF FAULTING

The existence of the faulting and of the overlying unconformity, and the characteristics of these features, are defined amply by the many boreholes and the extensive seismic data in the area (Fig. 3). The unconformity is established as being of middle Eocene age, with several criteria supporting this conclusion. Sedimentary rocks of this age are not present anywhere in the Cretaceous overthrust belt, yet directly east of the zone of thrusting about 500 m of middle Eocene Guayabal shale is present. The Coapa 1 well, which is just a few hundred meters, or less, east of the fault marking the eastern limit of the Cretaceous overthrust belt, penetrated 495 m of Guayabal shale with no coarse clastic rocks present in any of the Eocene section, nor with any evidence of a depositional or erosi nal hiatus within the Eocene. Because the overlying upper Eocene Chapopote shale extends continuously across the bounding fault, and the underlying lower Eocene Aragon beds are essentially identical both beneath the Guayabal and beneath the unconformity, the hiatus is established as being the age equivalent of the Guayabal Formation.

Upper Eocene to recent sedimentary deposits lie directly on the unconformity and progressively overlap it toward the west; the unconformity surface truncates various Cretaceous limestones as well as lower Eocene and Paleocene shales. The pre-unconformity rocks are folded and overturned locally, yet no rocks younger than early Eocene have been discovered beneath the unconformity.

In Alberta, Canada, the "foothills structures" described by various authors present geologic conditions somewhat similar to those in Veracruz. Although the thrusting in Alberta involves Paleozoic as well as Cretaceous rocks, and there are other dissimilarities, the Mesozoic part of the Alberta foothills as described by Dahlstrom (1970) and others appears to be closely related both in form and age to the Veracruz thrust belt.

BOUNDARY FAULT

Figure 5 shows the fault lines to be subparallel, with their eastern limit marked by a high-angle reverse fault. The latter delineates the eastern boundary of Cretaceous rocks in the area. It is interpreted to be the upturned edge of the lowermost

Fig. 5. Map of part of area near Angostura oil field showing traces of faults on middle Eocene unconformity.

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thrust (Fig. 2), and this thrust is postulated to be a sole fault, nearly vertical at its eastern limit and becoming a bedding-plane thrust on the west at depth. Horizontal movement along the fault plane may represent slippage of the indurated rocks of the Cretaceous along the softer red beds of the Jurassic. Farther west, in the state of Oaxaca, a similar thrust is suggested by surface geology. However, in the Veracruz area the stratigraphic position of the sole fault at depth is largely speculative.

TRANSCURRENT FAULTS

From purely theoretical considerations, the horizontal forces necessary to have produced the extensive overthrusting would not have been of equal intensity along the entire length of the disturbed area. Such variations in intensity of necessity would result in differential displacement of different segments of the fault plates. However, it is not plausible that broad, flat, relatively indurated limestone plates such as those present in this area could flex sharply parallel with the plane of their bedding. Therefore it was concluded that strike-slip faulting must be present to relieve the stresses produced. The seismic data confirm that such tear faults did develop. Designated as transcurrent faults, these are vertical faults which strike normal to the overthrust plates and are contemp raneous with the overthrusting. Similar faults are encountered in the Alberta foothills and elsewhere.

Fault A, along the north side of Angostura field (Fig. 5), is an example of this type of faulting, representing a large offset in the traces of the thrust faults. Fault A is a left-lateral strike-slip fault which does not appear to involve basement rocks and probably extends no deeper than the sole fault with which it is associated closely. It is one of 12 such tear faults which were recognized within the area.

The compressional forces producing the extensive overthrusting appear to have been at a maximum in the area south of Angostura field, between transcurrent faults A and D (Fig. 5). In this locality 18 imbricate plates are present, which is the greatest number present anywhere in the area. In addition, northwest of transcurrent fault B all the strike-slip faults mapped are left-lateral, and from fault C southeast all such faults are right-lateral, providing additional evidence that the maximum development of eastward-thrusting energy was in this central zone.

These transcurrent faults have approximately zero hade, yet exhibit little evidence of vertical movement. Horizontal slip commonly is extensive and quite varied between different fault plates along the same tear fault. On fault A for example, horizontal displacement of about 20 km is present in the eastern lowermost plate, whereas displacement of less than 5 km is indicated in the fault plate west of Angostura field.

The transcurrent faults, the thrust faults, and the bounding fault all appear to be contemporaneous. The thrust faults always are offset by the transcurrent faults, and the boundary fault is either offset, exhibits a change in strike, or both, where it is intersected by a transcurrent fault.

The presence of vertical faults has been recognized in the area for some time from studies of both seismic and well data. They were described originally as normal faulting. However, they apparently are tear faults representing lateral displacement but little or no vertical motion. Any vertical displacement across such faults is the result of juxtaposition of blocks having different structural elevations and does not result from direct vertical movement.

Additional evidence of transcurrent faulting is visible on aerial photographs of the Cretaceous outcrops in the mountains on the west. These reveal similar, transversely oriented faults, at least some of which appear to be projections or continuations of transcurrent faults identified in the subsurface.

In an extensive paper on the general paleogeography and tectonics of the Mesozoic of this area, Viniegra (1966) noted the existence of several very straight U-shaped valleys, particularly Acultzingo Valley west of Orizaba, Veracruz. These valleys are unique in that they have cross sections similar to those of glacial valleys, and are oriented normal to the structural and topographic grain of the area. Some investigators in the past considered them to be evidence of glaciation. However Viniegra suggested that they were the result of chemical erosion, that is, of solution along fractures. This hypothesis is substantiated by the present study. The orientation of the valleys, their appearance, even the inability to trace faults across the valleys, from one valley wall to the opposite, all are explained adequately if they are considered to be representative of erosional development along transcurrent faulting.

FRONTAL TROUGH

East of the fault which marks the eastern limit of the overthrust belt, and extending southeast along that fault for at least 60 km, is a graben averaging about 10 km in width. The east side of the graben is formed by a high-angle reverse fault which, like the normal faults which characterize the Tertiary basin on the east, appears to be a growth fault extending upward into the late Miocene

End_Page 384------------------------------

or younger beds and with throw increasing with depth. Contrary to the other Tertiary faults in the basin, which are normal faults downthrown to the east, this is a reverse fault downthrown to the west. The Loma Bonita anticline on the eastern upthrown side of the fault may represent a shale diapir.

The strike of the reverse fault is more north-south than the average strike of the bounding fault marking the west edge of the trough, resulting in a narrowing of the graben from north to south. Whether this reverse fault dies out southward or ultimately intersects the fault separating the Cretaceous overthrusts and the graben is not known. However, activity involving the fault on the east of the graben continued over a long period of time, with the probability that movement along this alignment was present at least as early as the beginning of the Tertiary, as suggested by seismic evidence. Apparently the graben and the associated reverse fault on the east were present during the middle Eocene when the major thrust faulting was going on in the west, and may have acted as a buttress t limit the eastward thrust of the Cretaceous limestone plates.

Within the graben at Novillero and Veinte fields (Fig. 3), gas production is obtained from Oligocene conglomerates which are folded gently into anticlinal structures. Seismic evidence suggests that more than 6,000 m of Tertiary sedimentary rocks are present in this trough. However, the deepest hole drilled in the graben is the Coapa 1; this well penetrated only 4,500 m of Tertiary shale containing but a few fine-grained sands from which gas is produced. It is bottomed in uppermost lower Eocene. It encountered no coarse clastic rocks but, as previously noted, did encounter 495 m of middle Eocene Guayabal shale. The shale, or any equivalent thereof, is not present in the west as shown by the many wells including several located only 5 to 6 km west of the Coapa 1.

Seismic evidence suggests that at least 2,000 m of lower Eocene and Paleocene sedimentary rocks are present below the depth reached by the Coapa well, but gives no indication of either the depth or type of the pre-Tertiary rocks which underlie the lower Tertiary strata.

GENETIC ENVIRONMENT

The well data and to some extent the seismic data provide information bearing on the environmental conditions in the middle Eocene during the time of orogenic movement. No single criterion of itself presents irrefutable evidence of the ambient conditions at such time, but the consensus of data strongly indicates that at least the eastern part of the imbricate zone and the accompanying unconformity developed beneath sea level, probably in deep water.

The faunal assemblages which are associated with both the overlying upper Eocene and underlying lower Eocene rocks are typical of a deep-water environment, including no shallow-water fauna except pelagic forms. The Foraminifera identified include predominantly such bathyal species as Cibicides robertsoniana, Planulina wuellerstorfi, Anomalina dorri, Cyclomina cancellata, as well as a few other similar genera. It is evident that movement of the sea floor from deep ocean to dry land thence back to deep water would have been required if the unconformity surface representing middle Eocene time had developed above sea level, and all this would have had to take place very rapidly, for the time span of the middle Eocene is geologically relatively short. It appears more probable that the thru ting occurred on the sea floor with the unconformity developing in deep water.

Additional evidence for such submarine genesis of the unconformity is present where the Cretaceous limestones are directly beneath the hiatus. Drilling has disclosed that the Cretaceous rocks consist, in places, of breccias containing massive blocks, often over 10 m thick and of unknown lateral extent, but much too large to have been transported any distance. They apparently have not been subjected to subaerial erosion, only to the less drastic action of marine currents. Such ocean currents also would have served to remove the crumpled debris of lower Eocene shales involved in the thrusting, and to redeposit this fine-grained material in the trench in front of the rising thrust plates.

The absence of subaerial erosion is demonstrated further by the angularity of the edges of some of the smaller limestone fragments present in cores taken in the breccia, and by the total absence of coarse clastic rocks in the middle Eocene deposits east of the zone of faulting. Although not necessarily positive indications of submarine conditions, these criteria do strongly suggest such an ambiance.

Coarse sedimentary rocks, nevertheless, are present locally in the upper Eocene and Oligocene both east of the overthrust zone and above it. They reach their greatest extent in the Oligocene where they act as reservoir rocks to produce gas at Novillero and Veinte fields. The conglomerates generate strong seismic reflections because of porosity and density contrast with the enveloping shales and on this basis can be traced westward by seismic means. Such reflections do not coalesce with the reflection representing the unconformity

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surface, indicating that the conglomerates do not impinge on that surface. This suggests that these upper Eocene and Oligocene clastic rocks are derived from source rocks farther west which were eroded as a result of vertical movement of the Sierra Madre Oriental subsequent to the overthrusting. They thus are not directly representative of the specific orogenic movement which produced the middle Eocene unconformity.

Today the middle Eocene unconformity in Veracruz exhibits an eastward dip ranging between 10 and 25°, but the configuration of the unconformity surface suggests that its original attitude may well have been more horizontal. A low-slope angle would be compatible with the theory that at least the eastern part of the unconformity developed below sea level, in deep water.

From these criteria it is possible to postulate a sequence of events during Eocene time in which expanding forces in the Sierra Madre Oriental, or west of it, caused extensive thrusting and imbrication of the Cretaceous and lower Eocene rocks during middle Eocene time while the rocks were under a considerable depth of water. Such orogeny, as it progressed, caused tilting of the area toward the east-northeast and elevation of the western part of the overthrust foothills belt. During very late Eocene and in Oligocene time this resulted in rapid erosion and deposition of conglomerates in the bordering seas east of or overlying the eastern part of the overthrust belt.

Subsequent subsidence is indicated by the gradual overlap of upper Tertiary beds toward the west onto the unconformity, but the uniformly fine-grained character of the clastic rocks which form the upper Tertiary throughout the Veracruz basin suggest that this tilting, uplift, and subsidence took place very gradually, continuing throughout most of Tertiary time. More recently, the area has been uplifted, accompanied by late Tertiary and Holocene volcanism in peripheral areas. Present-day earthquake activity suggests that related orogenic processes still continue.

HYDROCARBON ENTRAPMENT

Gas production is obtained in the Tertiary basin from basal Miocene and Oligocene sandstones and conglomerates, and both oil and gas are produced from the intensely overthrust Upper and middle Cretaceous limestones which compose the foothills of the Sierra Madre Oriental.

Until recently the only commercial production in the area of overthrusting was at Angostura, discovered in 1952, which has yielded prolific amounts of oil from breccias in the Upper Cretaceous Mendez limestone. At Angostura the fault plate which provides the oil reservoir exhibits reversal to the north, undoubtedly associated with major horizontal movement along the transverse fault bounding the north edge of the field. Additionally, rollover into the edge of the overthrust fault is present, and low-porosity, low-permeability upper Eocene shales provide an unconformable, impervious cover for the reservoir rocks. Although these shales could be source beds, the oil is believed to be of Cretaceous origin, not Tertiary, as in the Veracruz basin the Tertiary appears to generate gas exclusi ely.

For most of the fault-plate structures, the seismic data or drill data disclose an absence of one or more prerequisite trapping mechanisms. Rollover is lacking, inversion of plunge along strike is not present, or the sedimentary deposits overlying the unconformity are not sufficiently impervious to form a seal, thus explaining the dearth of hydrocarbon accumulation. A few small, isolated zones of oil or gas accumulation, such as at Nopaltepec, are present but these have such limited areal extent that they are not commercially attractive. They even may represent lateral migration of Tertiary gas into Cretaceous limestone reservoirs, in some cases.

One particular fault plate just west of Angostura (Fig. 5) exhibits wide and extensive anticlinal structure suggesting good productive possibilities. Unfortunately, all of the wells drilled on this feature have encountered fresh water (5,000 ppm or less of salt) rather than saline water, with only vestiges of oil, leading to the conclusion that this feature has been flushed out by invasion of meteoric water.

Most recently, oil and gas have been discovered north of Angostura at Mata Pionche, Copite, and Plan de Oro. Except for the last which produces from Cretaceous rocks directly underlying the unconformity at depths of about 500 m, production appears to be coming from various zones of secondary porosity well below the top of the Cretaceous, at depths of 2,500 to 2,800 m primarily from the middle Cretaceous Orizaba limestone. Fracture porosity may be a factor, but anticlinal structure, affecting the thrust plates and possibly predating the overthrusting, is of significant importance.

SEISMIC DATA

Much evidence bearing on the complex structural conditions in the Veracruz province was provided by the many exploratory holes in the area, as previously noted. However, detailed evaluation of the structural conditions was accomplished

End_Page 386------------------------------

primarily through study of the seismic data. These data vary from single-coverage analog sections to the most sophisticated, digitally recorded and processed presentations, and provide very detailed coverage (Fig. 3).

Although not particularly evident from the sample section shown in Figure 6, Tertiary information is usually good on the seismic sections, but reflections representing Cretaceous bedding planes are limited in extent or are absent. However all sections, like the illustration, show a great many events which are identified as diffractions.

Diffracted energy is defined as energy which is reflected from a subsurface source acting as a single point relative to the plane of the section. In this area conditions are ideal to produce diffractions, for the thrusting has resulted in high-velocity, high-density limestones subsequently being enveloped in low-velocity, low-density shales, yielding reflecting points of little lateral extent as seen by seismic lines normal to the strike, but having great acoustic contrast.

As a consequence of the preponderance of diffracted events and the scarcity of conventionally reflected data, the locations and characteristics of these many diffractions were used in preparing much of the interpretation. Not only did most of the diffraction sources line up in an ordered and meaningful manner but, with the help of the well-data available, the thrust plates which they bounded could be correlated from line to line. Use of diffracted energy as a means of developing a structural interpretation is unconventional.

CONCLUSIONS

The evaluation and analysis of the Cretaceous structures have demonstrated that careful integration of geologic and seismic data can resolve highly complex structural conditions even when the seismic data are obscure and the relations of the wells far from obvious. The precise dating of the middle Eocene orogeny in the Veracruz area may be of significance for other regions where comparable orogenic activity has occurred but has left a less well defined historical record.

Certainly the work involved has proved economically rewarding to Petroleos Mexicanos. The recent discoveries of new oil and gas production in the overthrust zone suggest that the economic potential of the area may be considerable and demonstrate that the effort required to decipher

Fig. 6. Representative seismic section near Angostura field.

End_Page 387------------------------------

such complex structural relations can be worthwhile.

References:

Dahlstrom, C. D. A., 1960, Structural geology in the eastern margin of the Canadian Rocky Mountains: Bull. Canadian Petroleum Geology, v. 18, p. 332-406.

Douglas, R. J. W., H. Gabrielse, J. O. Wheeler, O. F. Stott, and H. R. Belyea, 1970, Geology of western Canada, Chap. 8, in Geology and economic minerals of Canada, 5th ed.: Canada Geol. Survey Econ. Geology Rept. 1, p. 365-488.

Link, T. A., 1949, Interpretations of Foothills structures, Alberta, Canada: AAPG Bull., v. 33, p. 1475-1501.

Price, R. A., and E. W. Mountjoy, 1970, Geologic structure of the Canadian Rocky Mountains between Bow and Athabasca Rivers, a progress report, in Structure of the Southern Canadian Cordillera: Geol. Assoc., Canada Spec. Paper 6, p. 7-25.

Viniegra, F., 1963, Geologia del Macizo de Teziutlan y de la Cuenca Cenezoica de Veracruz: Asoc. Mexicana Geologos Petroleros Bol., v. 15, p. 101-163.

Viniegra, F., 1966, Paleogeografia y tectonica del Mesozoico en la Provincia de la Sierra Madre y Macizo de Teziutlan: Asoc. Mexicana Geologos Petroleros Bol., v. 18, p. 145-171.

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Acknowledgments:

(2) Seismograph Service Corporation.

(3) Petroleos Mexicanos.

Copyright 1997 American Association of Petroleum Geologists

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