Integrated Stratigraphic and Depositional-Facies Analysis of Parasequences in a Transgressive Systems Tract, San Joaquin Basin, California
Robert S. Tye, James S. Hewlett, Peter R. Thompson, David K. Goodman, 1993. "Integrated Stratigraphic and Depositional-Facies Analysis of Parasequences in a Transgressive Systems Tract, San Joaquin Basin, California", Siliciclastic Sequence Stratigraphy: Recent Developments and Applications, Paul Weimer, Henry Posamentier
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Oligocene-Miocene age Vedder and Jewett sands represent a retrogradational parasequence set forming a seismically defined, 492-m (1500-ft) thick transgressive systems tract (TST). The sandstones were deposited in a ramp setting on the southeastern margin of the San Joaquin basin, California. Due to basin subsidence, underlying nonmarine highstand systems tract (HST) Walker Formation sediments were transgressed and overlain by marine and nonmarine deposits of the Vedder and Jewett sands. Seismic, wireline-log, and core data indicate that parasequences in the upward-deepening TST comprise alluvial, fluvio-deltaic, and shallow-marine facies; however, the facies composition of the parasequences changes within a definite and predictable stratigraphic pattern. Basal parasequences in the TST contain a large proportion of nonmarine facies (alluvial fan, fluvial, flood plain), whereas stratigraphically higher parasequences (just below the maximum-flooding surface) comprise shallow-marine facies (estuarine, lagoonal, washover, marsh). Progradation of each parasequence was terminated by a marine-transgressive episode.
The TST is manifested as a succession of backstepping and onlapping seismic reflections. Parasequence boundaries and other stratigraphic discontinuities identified from wireline-log and core data are seismically detectable because of impedance contrasts between marine and nonmarine strata at marine-flooding surfaces. Mappable reflections correspond to (1) the basal sequence boundary, (2) marine-flooding surfaces, and (3) the maximum-flooding surface. Amplitude and phase variations imply depositional facies changes within parasequences laterally along individual reflections. Amplitude decreases coincide with a sandstone-to-shale facies change within a parasequence and therefore are a good estimate of lithosome continuity.
Parasequences and their bounding surfaces in the TST have diagnostic sedimentologic and biostratigraphic signatures. Low benthonic foraminiferal abundances characterize shallow-marine deposition; abundances increase to a maximum at the seismic downlap surface (maximum-flooding surface). Dinoflagellate occurrences accurately record the transgressive episodes, especially in the predominantly nonmarine lower TST.
The recovery of 657.5 million barrels of oil (MMBO) and 221.9 billion cubic feet of gas (BCFG) from Vedder and Jewett reservoirs is strongly related to sandstone evolution, stratigraphy, and structure. Interpretations and concepts developed in this study will improve parasequence delineation and prediction of reservoirs, seals, and traps.
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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.”