Early in the development of Coalinga East Extension, Pleasant Valley, and Guijarral Hills fields of California, geologists recognized that numerous permeability variations and traps exist in the Eocene Gatchell sand. These anomalies developed as the sand became plugged with interstitial kaolinite. Kaolinite can be detected easily in cores but is extremely difficult to recognize from electric logs. Kaolinite forms in the Gatchell as a postdepositional alteration product of feldspars, micas, and formation water. Pore spaces filled with hydrocarbons, however, are not subject to this reaction; thus porosity and permeability are preserved. Subsequent structural adjustments could leave these “clay-sealed” oil traps in seemingly unfavorable structural positions.

Permeability traps in the Gatchell are too numerous for all of the oil in the Coalinga area to have been found. A thorough understanding of existing production as well as structural, depositional, and diagenetic history is required to find the remaining reserves.

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