Sequence stratigraphy is the study of genetically related strata-depositional sequences-which are bounded by unconformities or their correlative conformities. An unconformity is defined as a sedimentary structure of regional occurrence in which two groups of rocks are separated by an erosional surface; the erosion may be by subaerial or submarine processes.

A sequence stratigraphic analysis of an oil-producing basin has several components. First and foremost is establishment of a geologic model in which facies distribution and thickness in regressive-transgressive cycles are reconstructed by use of subsurface well data, paleontologic data, seismic, and outcrop sections where available. Emphasis is placed on location of the shoreline facies for each time-stratigraphic interval. Second, key surfaces related to unconformities or condensed sections are traced regionally. Third, the distribution of the coarsest-grained detrital sediment (e.g., conglomerates) is analyzed in relation to unconformities. Fourth, condensed sections, generally with high total organic content (source rocks), are mapped and related to the above features. Burial history of the basin is then related to oil generation and migration to traps.

The tracing of key surfaces in the shoreline and shelf (neritic) setting is essential. Two types of major erosional surfaces are observed within or at contacts of sandstone units. Each is associated with major changes of sea level, but the magnitude of erosion may be influenced locally by tectonic events. One type, a sequence boundary, is called a lowstand surface of erosion (LSE) related to a lowering of base level which causes a subaerial exposure and incisement of drainages into older deposits. The second type is a transgressive surface of erosion (TSE, also called a ravinement surface) related to shoreline and shoreface (marine) erosion, that is related to a rising sea level and water deepening. The unconformity associated with the TSE occurs within a depositional sequence. These two surfaces may merge in the offshore marine or interfluve areas between paleodrainages.

A third type surface can sometimes be identified that is related to nondeposition with possible minor erosion. If present, it occurs within a marine condensed section that

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generally has a high total organic content. Minor scour may concentrate lags of shells, or glauconite and phosphate grains. Bentonite may occur in shale layers above or below the surface. In sequence stratigraphy such a surface has been called: the surface of maximum transgression, maximum flooding surface, or, maximum starvation surface.

Minor erosional surfaces associated with depositional processes within environments of deposition are called diastems (e.g. scour at the base of a channel). Diastems are not to be confused with the major erosional surfaces.

Tracing the key surfaces from shallow water into deeper marine environments of the slope and basin and across growth faults sets the stage for predicting sand distribution and offers a predictive model for improved success in future exploration and production programs.

New stratigraphic terminology has been introduced to subdivide and analyze depositional sequences. These new terms have been classified as chronostratigraphic, and offered as replacements for lithostratigraphic or allostratigraphic terms as defined by the North American Commission of Stratigraphic Nomenclature (e.g. parasequence for formation, member or bed). Geologists now face the challenge of integrating the new terminology with the formalized standard terms, or of ignoring most of the new terms as unnecessary, or of adopting the new terms and discarding the old.

Examples of sequence stratigraphic analyses or analyzing petroleum occurrences are discussed in different tectonic settings.

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