Recent Applications of Siliciclastic Sequence Stratigraphy
The application of sequence stratigraphic concepts, first proposed during the mid-1970s, has led to both successes and failures in petroleum exploration. Although a broad spectrum of reservoir occurrences are predicted in siliciclastic sequence stratigraphic models, the most successful applications have been in incised valley fills (lowstand and transgressive systems tracts) and turbidite systems (lowstand systems tracts). Notable exploration failures have also occurred in these settings. We analyze one example from each of these depositional settings to illustrate the successes and limitations of sequence stratigraphy in exploration.
The first example is from the Pennsylvanian Morrow Formation of southeastern Colorado. This is an incised valley fill trend that produces from several different valley fill systems. Of the 181 wells drilled in the Clifford to Siaana fields trend, 41.4% of the wells penetrated the valley (75 wells), 19.3% of the wells found reservoir-quality sandstone (35 wells), and nine new fields were discovered (5% rate of success). Since 1985, when the play was first interpreted as an incised valley fill, drilling statistics indicate no significant increase in the success rate of discoveries, even though a greater percentage of wells penetrated valley fill facies. This case study indicates that the major problems in valley fill exploration were (1) locating the valley, (2) delineating reservoir sandstone within the valley, and (3) finding a trap for the reservoir.
Exploration by Soekor (Ltd.) Pty in the Lower Cretaceous turbidite systems of the Bredasdorp Basin in offshore South Africa is ideal for demonstrating the frontier exploration applications of sequence
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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.”