Sequence Stratigraphy of Petroleum Plays, Post-Rift Cretaceous Rocks (Lower Aptian to Upper Maastrichtian), Orange Basin, Western Offshore, South Africa
A. Muntingh, L.F. Brown, Jr., 1993. "Sequence Stratigraphy of Petroleum Plays, Post-Rift Cretaceous Rocks (Lower Aptian to Upper Maastrichtian), Orange Basin, Western Offshore, South Africa", Siliciclastic Sequence Stratigraphy: Recent Developments and Applications, Paul Weimer, Henry Posamentier
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The interpretation of viable siliciclastic lowstand petroleum plays and fairways within post-lower Aptian (112–68 Ma) Cretaceous passive-margin rocks underlying 90,000 km2 of the Orange Basin, offshore South Africa, was carried out using seismic- and sequence-stratigraphic concepts and methods. Twenty-three fundamental third-order, type 1 depositional sequences and five composite third-order, type 1 sequences comprising 12 fourth-order sequences were identified, provisionally dated, correlated, and mapped.
The sequences compose four third-order progradational and/or aggradational sequence sets (second-order supersequences) bounded by second-order, type 1 unconformities. Control of the stacking patterns of third-order sequence sets was probably an interplay between basin subsidence and first- and second-order eustatic sea level cycles. During the deposition of third-order, oblique progradational sequence sets (second-order "lowstand systems tracts"), the component third-order lowstand systems tracts shifted far into the basin. During the deposition of a third-order, highly sigmoidal, aggradational set (second-order "highstand systems tract"), however, component third-order highstand systems tracts were stacked along the outer shelf. Consequently, slumping and gravity faulting of distal fine-grained prodeltaic and slope facies were common and basinward shifts of component third-order lowstand tracts were highly constrained. A positive aspect of the aggradational third-order sequence set, however, is its potential for supplying large volumes of reservoir-quality sands to basin-floor fans during lowstand fluvial entrenchment of the third-order highstand fluvial/deltaic systems tracts.
The most favored petroleum plays and fairways are restricted to lowstand systems tracts within highly progradational, third-order sequence sets. These plays comprise third-order incised valley-fill, prograding (deltaic) wedge, and basin-floor prospects that shifted far into the
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Applying depositional sequence stratigraphic concepts to the interpretation of siliciclastic depositional systems is becoming an increasingly important tool in petroleum geology. After a succession of breakthroughs during the 1970s and 1980s, sequence stratigraphic concepts now have entered a phase of intense application and documentation, especially with regard to successful implementation in the field of petroleum geology. Workers have applied these concepts to a variety of databases, ranging from outcrop to cores to electric logs and to multifold seismic data. Clearly, sequence stratigraphic concepts embody–not a rigid model or template–but rather a way of looking at geology. This volume has two purposes: to compile some recent applications of siliciclastic sequence stratigraphic concepts, and to present new studies focused on refining conceptual models. This memoir grew out of a 1991 symposium, "Variations in Depositional Systems Within a Sequence Stratigraphic Framework: Applications to Exploration," organized by the authors at the AAPG annual meeting at Dallas. Robert Loucks and Rick Sarg have edited a companion volume, also published by AAPG,“Entitled Carbonate Sequence Stratigraphy: New Developments and Applications.”