sandstone - medium grained gray - 4" diameter oil well core section from the Stevens Sand

Oil well 4" diameter core section from the Stevens Sand, a prolific Upper Miocene oil producer in California's San Joaquin Valley. This medium-grained sandstone is composed primarily of quartz with feldspar and some biotite. The Stevens sand is derived from the rapid uplift of the Sierra Nevada during the Miocene. 

A plug was occasionally removed from each core for determination of permeability and porosity of the sand. Well data has been lost for this core, but the Stevens Sand is roughly 12,000 feet below the San Joaquin Valley floor. In 1991, ARCO completed a well in the Stevens, turning the well from vertical to horizontal at 11,253 feet below the surface and then drilling a 2252 foot lateral. This well produced at three times the rate of vertical wells in the Yowlumne Field. The technology involved is impressive.

The Yowlumne is one of a series of giant light oil fields that produce from the Stevens. This core is an interesting example of how petroleum geologists learn about the rocks they are drilling through, and a practical size for the classroom. 

Usually a formation can be identified from the drill cuttings washed from the drilling mud, but occasionally a coring bit is used to produce continuous core. Typically, sections up to three feet long are stored in core trays and boxes, and core libraries allow a view of rocks that might not be locally exposed for study at the surface.

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