McJANNET, G. S., U.S. Department of Energy, Elk Hills, Tupman, CA

ABSTRACT: Overview of Geology, Production, and Reservoirs at Elk Hills

Elk Hills oil field, Naval Petroleum Reserve 1, a billion barrel giant, is located in the southwest portion of the San Joaquin basin. It displays the classic surface expression of a large, single, anticlinal structure approximately 7 mi wide and 17 mi long. In the subsurface, however, it is characterized as an anticlinorium with three distinct and separate large oil and gas productive structures with multiple reservoirs of varying rock types: the Northwest Stevens anticline, the essentially parallel 29R anticline to the south, and the major west to northwest-trending 31S anticline. This latter structure appears to bifurcate to form the North and South Coles Levee structures. These structures are the result of compressional and wrench deformation within the en echelon fold thrust belt ssociated with the San Andreas fault in the southwest San Joaquin Valley. The intensity of deformation decreases eastward from the San Andreas fault.

More than 24,400 ft of sediment have been penetrated within Elk Hills field. Commercial production has been established only within the upper 10,000 ft of the section. The four major productive units include the lower Miocene Carneros, the upper Miocene Stevens zone of the Monterey Formation, and the Pliocene Shallow Oil zone and Dry Gas zone. Numerous hydrocarbon shows have been encountered in the Oligocene, Eocene, and Cretaceous rocks, indicating that oil and gas generation limits have not been reached.

Upper Miocene large, sand-rich turbidite systems with an eastern source deposited the prolific Stevens sand reservoirs in the eastern Elk Hills. Other smaller and more active channel turbidite systems were sourced from the passing Salinian block to the west and developed between actively growing subsea structures of western Elk Hills. Shallower Pliocene Calitroleum and Gusher sands and other Shallow Oil zone and Dry Gas zone sands were deposited as the basin filled. Continuous to episodic deformation initiated in the middle Miocene resulted in the structures as we know them today.

An active development and exploratory program that began in 1974 has resulted in the drilling of more than 1183 wells through 1992. Improved oil recovery techniques have been initiated in all Stevens zone reservoirs (peripheral water flooding, minor pattern flooding, gas injection/pressure maintenance, and horizontal wells for wedge oil). Steam flooding, water flooding, and gas injection for pressure enhancement are being used in the Shallow Oil zone. With present technology and effort, Elk Hills is on it's way to the second billion barrel mark!

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