Model of Fluvial Deposition for Control of Oil Migration and Entrapment in Upper Eocene to Oligocene Sespe Formation, West Montalvo Field, Ventura County, California

R. Kumbe Sadler

The Sespe Formation consists of continental red beds deposited during the tectonism that resulted as the Pacific-Farallon spreading ridge approached the North American plate. The Sespe at West Montalvo field is over 7,000 ft thick and consists predominantly of fine to medium-grained sandstones interbedded with siltstone and mudstone deposited in the central part of the Oligocene basin. Oil production was established in 1951 from the upper 2,000 ft, known as the Colonia zone. The Colonia zone has been subdivided into six sandstone packages 350-600 ft thick, based on a model of laterally migrating fluvial systems that created local intraformational unconformities. These systems had unique depositional characteristics that can be inferred from well-log analysis and related t facies described in outcrops surrounding the Ventura basin. These characteristics include sandstone to shale ratios, relative bed thicknesses, lateral continuity of sandstone and shale interbeds, and whether the sandstone beds exhibit normal or reverse grading, or have sharp bases and tops. The fluvial environments include small braided distributary streams, larger trunk streams, and broad shallow braided streams. All of the sandstone packages contain oil-bearing beds, but the package in which the sediments were apparently deposited in a system of broad, shallow braided streams is the most oil-prone. These sandstones are relatively thin, have sharp bases and tops, and are laterally continuous across parts of the field. The rapid sedimentation and the cut-and-fill processes of braided st eams may have created interconnecting fluid pathways that allowed the migration and updip accumulation of oil.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.