PETROLEUM GEOSCIENCE OF THE SOUTHERNMOST SAN JOAQUIN BASIN, CALIFORNIA

Two petroleum exploration plays are currently active in the southernmost San Joaquin basin. Trap types in the two plays are markedly different. One play is stratigraphic in nature, and the other, the Upturn play, combines dramatic structure with stratigraphic elements.

Most traps in the basinal stratigraphic play result from pinchouts of mildly dipping upper Stevens submarine-fan sands. Well and 3-D seismic data indicate that these fans are very elongate and prograded to the northwest. Their distribution was controlled by syndepositional structuring and relict paleobathymetry due to differential compaction over older fans.

The Upturn structural/stratigraphic play lies in a 25-mile long trend of nearly vertical beds at the southernmost edge of the San Joaquin basin. These steeply dipping beds comprise the front limb of a late Cenozoic fault-propagation fold localized along the previously formed White Wolf fault system. Footwall traps beneath reverse-separation faults are a common mode of Upturn dip closure. As in the basinal play, Monterey and Reef Ridge sands are concentrated on the downthrown sides of syndepositional faults, enhancing footwall reservoir thickness and hence prospectiveness. Sand pinchouts, tear faults, and folding above lateral ramps provide strike closure.

Oil in both plays derives from early mature Monterey source rocks that have been generating in late Pliocene and Quaternary time. We interpret that oil migration pathways, both bed-parallel and across bedding, exert control on field size. Migration maps show that bed-parallel pathways have varied dramatically since generation began. In summary, a large and diverse set of factors influences the distribution of known oil fields in the southernmost San Joaquin basin. Mapping of these factors highlights specific areas and ideas for future drilling.