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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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