The Shallow zone at Elk Hills was discovered in 1919; the Stevens zone was discovered by Standard in 1941. In 1944, the Navy and Standard lands were joined in a unit with the Standard as operator, and Congress authorized drilling of undeveloped parts of the Reserve to increase the production from 15,000 to 65,000 barrels per day. At the end of the war, production was cut back to the former figure which has subsequently been reduced to a current production of about 5,000 barrels per day from the Shallow zone and 3,000 barrels per day from the Stevens. Since the war, exploratory and development programs have been carried on.

The present known stratigraphic column at Elk Hills extends from the Pleistocene Tulare formation to the lower Miocene Vaqueros. Shallow-zone production is established in sands near the base of the Pliocene San Joaquin and in the upper 600 feet of the Pliocene Etchegoin formations. Stevens zone production is established in sands and fractured shale in the upper Miocene McLure formation. No commercial production has been established below the Stevens, but formation tests of the lower Miocene Carneros sand have shown the presence of gas and some oil.

The Shallow-zone structure is a broad faulted anticline similar to the surface structure. At the top of the Stevens, however, the structure consists of three en echelon folds, the larger eastern fold being fairly symmetrical and the two western structures asymmetrical with nearly vertical northern flanks. Evidence indicates at least three periods of structural movement during and since upper Miocene time.

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