DUBOIS, DEAN P., TIMOTHY L. KIRST, and JONATHAN D. PETERSON, Mobil Exploration and Producing U.S. Inc., Bakersfield, CA

ABSTRACT: Tulare Formation Permeability and Fluid Variations, South Belridge Field, Kern County, California

South Belridge field lies on a northwest-southeast-trending asymmetric anticline in the western San Joaquin basin. Reservoir sand geometry and fluid compositions in the unconsolidated Pleistocene Tulare Formation exhibit variations which reflect a complex relationship between structural control of fluvio-deltaic sedimentation and structural-stratigraphic control of oil migration and subsequent fluid movements.

Structural growth during deposition of the Tulare inhibited sand deposition across the anticlinal crest, resulting in overall thinning and reduced sand-body permeability.

Oil migration into the Tulare appears to have occurred during sedimentation as evidenced by deposits of highly degraded oil (bitumen). These deposits are believed to be fossil surface oil seeps and are concentrated in the lower Tulare on the northeastern flank of the current anticlinal crest. It is thought that the seeps occurred on or near the paleocrest and were subsequently rotated into their present flank position following axial tilting with further structural growth. Later oil migration pathways shifted to other areas.

The general path of migration appears to be near-crestal vertical leakage from the underlying lighter oil accumulation in the Miocene Monterey with subsequent vertical and lateral movement away from the crest in the Tulare. One such area showing current vertical lighter oil migration has been identified and is being exploited. Oil degradation during and after migration generated variations in oil gravity and viscosity. Oil sweep during steamflooding has followed depositional trends, and may also be influenced by pre-existing viscosity variations. Recognition of both influences upon steamflood behavior has resulted in a better understanding of areal performance differences.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.