Origin, Migration, and Accumulation of Oil in a Giant Oil Field, Wilmington Field, Los Angeles Basin

Chung-I Lee

Wilmington field has recoverable oil reserves of about 3 billion bbl, or 32% of the total reserves of the Los Angeles basin. Understanding the origin, migration, and accumulation of oil in the Wilmington field may provide a model for oil exploration in California. One of the most important factors determining Wilmington accumulation appears to be highly efficient oil expulsion and migration from the fractured shale source rock. The major source of Wilmington oil is believed to be the underlying Miocene Mohnian E shale; such a local source indicates that the migration distance was short. Organic geochemistry studies support this interpretation.

The Wilmington anticline developed during the early Pliocene from the late Delmonian to early Repettian. This was followed by the initiation of significant oil generation. Major faulting was initiated during the early Repettian with renewed faulting throughout the Repettian. The E shale acted as both a source rock and a fractured shale reservoir. Because of fracturing, the expulsion and migration efficiency of oil must have been very high, perhaps 80-90%.

The oil expelled from the E shale migrated along faults and fractures and was trapped in the overlying reservoir. The faults were then sealed by deposition of bitumen and minerals. This conclusion is supported by the presence of fractures filled with bitumen and calcite in cores from the E shale. However, field evidence indicates that oil production and associated decrease in reservoir pressure and subsidence caused the faults to leak.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.