Paleoenvironments and Sequence Biostratigraphy: Analysis of Eocene-Oligocene Rocks from the San Emigdio Mountains, Southern San Joaquin Basin, California

Peter R. Thompson, Steven R. Fekete, Martin B. Lagoe

Late Cenozoic structural uplift of the southern end of the San Joaquin Basin exposes an east-west oriented, westward-deepening belt of nonmarine to deep marine, Eocene-Oligocene rocks in the San Emigdio Mountains. This area is a major petroleum province, producing from multiple reservoirs in a complex structural and stratigraphic setting. Seismic approaches to sequence analysis are difficult due to the rugged terrane, structural complexity and nature of the basin fill. The paleoenvironmental and sequence relationships of these rocks, traditionally assigned to the Tejon, San Emigdio, Pleito, Tecuya and Temblor Formations, are established from a variety of surface and subsurface information. A sequence biostratigraphic approach emphasizes integrating microfossil distributio s with information on sediment composition, wireline log response, and other paleontological information in order to characterize systems tracts. Paleoenvironmental analysis is also based on benthic foraminiferal biofacies distributions. Proper stratigraphic correlations can only be correctly carried out within a well established paleoenvironmental framework, particularly in this area of rapid lateral facies changes.

Five sequences are recognized on the basis of wireline log correlations and sequence biostratigraphy: U, N, R, LZ, and UZ, ranging in age from middle Eocene to late Oligocene. Within each sequence the following features can be characterized by sequence biostratigraphic analysis: systems tracts and associated condensed sections, transgressive surfaces and depositional sequence boundaries. The different sequences exhibit varying distributions and characteristics throughout the area. In the eastern San Emigdio area the younger R, LZ and UZ sequences (latest Eocene to late Oligocene) only exhibit transgressive (TST) and highstand (HST) systems tracts. Evidence of lowstand deposits (LST) are found in massive, deep-water sands deposited in the western San Emigdio area. The older sequences, and N (middle Eocene to late Eocene), exhibit a full suite of systems tracts in the east (LST, TST, HST) and a preponderence of deeper water deposits (mostly LST, TST) in the west. The distribution and character of these sequences control the relationships between reservoir and seal facies, which in turn are the primary controls on hydrocarbon traps in this area, although these traps are often modified by structural deformation.

The sequence biostratigraphic approach improves prediction of such reservoir-seal couplets and allows for a more rigorous evaluation of play concepts.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995