--> Abstract: Seismic Stratigraphy of Depositional Sequences: High Resolution Images from a Passive Margin Slope Setting, Offshore West Africa, by R. T. Beaubouef, T. R. Garfield, and F. J. Goulding; #90933 (1998).
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Abstract: Seismic Stratigraphy of Depositional Sequences: High Resolution Images from a Passive Margin Slope Setting, Offshore West Africa

Beaubouef, R. T. & T. R. Garfield - Exxon Production Research Co.; F. J. Goulding - Exxon Exploration Co.

Seismic stratigraphy is a method developed in the 1970s for identifying reservoir, source and seal lithologies in the subsurface. Based on reflection characteristics, this methodology allowed assemblages of strata to be interpreted within a hierarchy of chronostratigraphically significant seismic surfaces. The unconformity-bounded depositional sequence was recognized to be the fundamental stratigraphic unit and its formation was related to eustatic changes in sea-level. Unique associations of stratal geometries were interpreted as highstand (HST), transgressive (TST) and lowstand (LST) Previous HitsystemsNext Hit Previous HittractsNext Hit; the building blocks of depositional sequences. Today, seismic stratigraphy remains a principle tool for lithologic prediction in the subsurface. While many basic tenets of the sequence stratigraphic model have been validated, modern seismic images are stimulating development and testing of new models and concepts. Important enhancements have been made through studies of passive-margin, slope settings in offshore West Africa where 3D seismic and well data allow stratigraphic analysis of Tertiary deposits at very high resolution. New observations from these data suggest that 1) the single 3rd order (1-3 m.y.) sequence, as defined by Vail, Mitchum, et al., 1977, can often be interpreted as a composite sequence comprised of high frequency sequences and associated Previous HitsystemsNext Hit Previous HittractsNext Hit and 2) common vertical stacking patterns within deep-water successions occur at a range of scales and can be related to interpreted changes in accommodation on the coeval paleo-shelf.

Seismic stratigraphic analysis of many West African deep-water reservoir intervals reveal multiple orders of depositional cyclicity, interpreted to have resulted from superposition of both long- and short-term cycles of relative sea level change affecting the Previous HittypesNext Hit, rates and volumes of sediment delivered to the slope from the shelf. Reservoir and seal intervals are placed within sequences and associated Previous HitsystemsNext Hit Previous HittractsNext Hit, sequence sets and composite sequences. Individual sand-rich, submarine fan deposits are interpreted as LST of high frequency sequences and may contain one or more genetically related depositional Previous HitsystemsTop. These are separated from one another by mud-rich hemipelagic and low-density turbidite deposits considered to represent the TST- HST of these sequences. Sequence sets are systematically stacked, high frequency sequences bounded by prominent, compound erosional surfaces. Sandstone-rich sequence sets dominated by the LST of high frequency sequences are termed LST sequence sets while mudstone-rich sequence sets dominated by the TST-HST of high frequency sequences are termed TST-HST sequence sets. Composite sequences are major unconformity bounded units of regional extent that contain a lower reservoir-bearing interval and an upper seal-prone interval corresponding to LST and TST-HST sequences sets, respectively.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil