SEQUENCE STRATIGRAPHY OF THE LATE NEOGENE, SOUTHERN SAN JOAQUIN BASIN, CALIFORNIA

This study provides a seismic-scale sequence stratigraphic framework for the 1300-m late Neogene succession of the Bakersfield Arch, southern San Joaquin basin, California. Within the context of time-significant surfaces, an evaluation is presented of the shallow marine siliciclastic strata of the Etchegoin (late Miocene to Pliocene) and San Joaquin (Pliocene) Formations which have produced 10.4 billion m3 (367 billion ft3) of dry gas adjacent to the study area.

Three-dimensional seismic data allow seismic-scale (50-150 m) interpretation of eight unconformity-bounded stratigraphic sequences. These sequences prograded and thickened westward/southwestward to fill the basin from east/northeast to west/southwest. Each sequence contains northwestward-onlapping transgressive systems tracts, separated by a flooding surface from overlying southwestward-downlapping highstand systems tracts. A third systems tract, the lowstand wedge, is represented by incision (widths of 180 – 600 meters) on uppermost highstand deposits. Each transgressive and highstand systems tract contains 30- to 80-m thick parasequence sets with characteristic stacking patterns composed of 6- to 20-m thick parasequences. Well log data enable interpretation of reservoir-scale (meter) parasequences and parasequence sets within each sequence. Limited core data reveal that one prolific gas-bearing unit was deposited within a wave-dominated tidal channel setting.

Rapid subsidence plus high sediment flux in the Bakersfield Arch area resulted in the preservation of systems tracts and the compartmentalization of basinward lowstand wedge facies. Pliocene deformation of the basin resulted in a distinct change in depositional style from coarsening- and fining-up parasequences capped by shales to a continuous series of coarse-based, fining-up sandstones, seen at both seismic- and reservoir-scales within sequence ‘G’. Following this change, shallow-marine facies were replaced upsection by nonmarine facies of the Pleistocene Tulare Formation.

This study of the integrated effect of tectonics, relative sea level, and sediment flux, in the context of a sequence stratigraphic framework, will enable better prediction of the geometries and facies distributions of gas-prone Neogene sands in the San Joaquin basin.