There are three known areas on the west side of the San Joaquin Valley where oil is produced exclusively from fractured rocks. These are in the South Belridge, Elk Hills, and Sunset oil fields. In each accumulation the structure is anticlinal; the fractured rocks are shale of upper Miocene age. The physical characteristics of the fractured rocks are not well known generally. The upper Miocene contains considerable brittle siliceous shales on the west side of the San Joaquin Valley which are known to have favorable reservoir characteristics in some places where tested. The distribution of these members is discussed.

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