Seismic-Stratigraphic Interpretation of Depositional Systems: Examples from Brazilian Rift and Pull-Apart Basins1
L. F. Brown, Jr., W. L. Fisher, 1977. "Seismic-Stratigraphic Interpretation of Depositional Systems: Examples from Brazilian Rift and Pull-Apart Basins", Seismic Stratigraphy — Applications to Hydrocarbon Exploration, Charles E. Payton
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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.
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Papers from a research symposium at the 1975 American Association of Petroleum Geologists and supplemented by later reports became “Seismic Stratigraphy Applications to Hydrocarbon Exploration”, one of AAPG’s best-selling book publications. Dramatic improvements in seismic imaging were demonstrated, a result of developments in seismic data quality and the processing capability of electronic technology. Twenty-eight articles are grouped into three sections. The first describes principles that both permit and also limit interpretations. The second section presents sixteen articles that describe the qualitative approach to stratigraphic interpretations of reflection records, and the final section presents techniques and examples of modeling. Of particular interest are a series of eleven papers in the second section under the subject heading of “Seismic stratigraphy and global changes of sea level”. Prepared by P. R. Vail, R. M. Mitchum and others from Exxon, they describe the regional unconformities and stratigraphic changes resulting from sea level fluctuations, and the manner in which these changes can be interpreted from seismic surveys. For many individuals within the oil industry who purchased this book, it was their first introduction to the modern concept of sequence stratigraphy that would have a major impact on the methodology of petroleum exploration.