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The upper Miocene Monterey shale units of the Midway-Sunset field and the San Joaquin basin are the main source rocks for many of the oil producing formations. The Monterey Formation is derived from diatom frustules making the rocks rich in biogenic silica. The four siliceous members of the Monterey Formation in Midway-Sunset are the McDonald, Antelope, Belridge, and Reef Ridge shales with the entire section being over 5000 feet thick and interbedded with the Williams, Republic, Spellacy, and Potter sandstones. These siliceous rocks are found as unaltered amo rphous opal-A, and its diagenetic equivalents of opal-CT and quartz. The rock is characterized by porosities ranging from 70-10% and permeabilites less than 0.1 millidarcy. Since 1978, the members of the Monterey Formation have become major producing reservoirs on the west side of the San Joaquin basin as a result of hydraulic fracturing and/or cyclic steaming. Commercial production was established at Midway-Sunset by Santa Fe Energy Resources in 1985 in the Reef Ridge Shale and to date over 160 wells have been drilled to develop this reservoir. At least seven other areas in Midway-Sunset have had drilling and completion attempts aimed towards establishing commercial potential with mixed results to date from Miocene Shales.

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