General Overview of the Elk Hills Field
Authors: McJannet George S. 
Published: December 2007

Abstract
The billion-plus barrel giant Elk Hills oil field, Naval Petroleum Reserve No. 1 (NPR-1), is located in the southwestern portion of the San Joaquin basin, California. The imposing single anticlinal surface feature of the field is dominated in the subsurface by three distinct Stevens (Late Miocene) depth oil and gas producing structures: the Northwest Stevens anticline, the 29R anticline, and the 31S anticline. Each structure contains multiple reservoirs with varying rock types that require individualized engineering treatment.
Of the more than 24,000 feet of sediment beneath Elk Hills, commercial production has been established in the upper 10,000 feet. The four major productive zones include the lower Miocene Carneros Sandstone, the upper Miocene Stevens sands and siliceous shales of the Monterey Formation, the Pliocene Shallow Oil Zone (SOZ) and the Dry Gas Zone (DGZ) of the Etchegoin and San Joaquin Formations. The Stevens sands are further divided into the sand rich Main Body "B" (MBB)/Western 31S (W31S) reservoirs deposited by the turbidite fan system sourced from the east, the smaller 26R and 24Z turbidite channel systems and the Northwest Stevens (NWS), sourced from the west.
The numerous types and sizes of reservoirs often require new and unique development and improved oil recovery techniques. For example, horizontal drilling along with crestal gas injection in the steeply dipping 26R reservoir takes advantage of gravity drainage in the thick, complex, layered, deep marine, turbidite channel sands. An expanding gas cap which resulted in a shrinking oil band above a stable oil/water contact made this reservoir the ideal candidate for horizontal drilling.
Successes with the 26R horizontal wells prompted the drilling of a horizontal well in the Shallow Oil Zone to recover oil gravity draining oil along the south flank of the 31S Structure. Innovative liner replacements, drilling new wells, and gas injection into the low pressure Shallow Oil Zone have reversed the production decline.
The Stevens shales historically have been overshadowed by successes in the Stevens sand reservoirs such as waterfloods, infill drilling, and gas injection/pressure maintenance projects. Recent improved reservoir characterization studies indicate that the original oil and gas in place in the shales exceeds the original oil and gas in the sands. Simulations and selected tests have indicated that gas cycling and infill drilling in the N/A and C/D shales are the initial steps required to recover these significant reserves.
Other potential projects include EOR projects such as CO2 flooding in the Stevens shales, pattern waterflooding and selective infill drilling in the MBB waterflood projects, and horizontal drilling in the Northwest Stevens.
Results of these innovative techniques and recent studies have substantially increased hydrocarbon reserves in recent years.

Introduction
The Elk Hills oil field is located southwest of Bakersfield, in Kern County California (Fig. 1). It has had a colorful and interesting past, and now has a unique future. Early oil seeps and spectacular discoveries in the early 20th century in areas within and adjacent to Elk Hills led to the withdrawal of public lands in Elk Hills to all forms of entry on September 27, 1909 by President Taft. This was done on the recommendation of the Director of the U.S. Geological Survey.