The main source of oil production in California is in sand reservoirs. In recent years, however, production from fractured rock reservoirs has gained prominence. Fractured rock reservoirs can in general be divided into two groups: fractured cherts and shales in sedimentary beds and fractured metamorphosed basement rocks. Approximately 55,000 barrels of oil per day are being produced at present from fractured rock reservoirs. It is estimated that 15,000 barrels per day are being produced from fractured basement rocks and 40,000 barrels per day from fractured cherts and shales.

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