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Diagenesis of Oligocene and Eocene sandstones at the Greeley oil field, southern San Joaquin Basin, California
The Vedder Formation (Oligocene) and Eocene sands at the Greeley oil field consist of arkosic to subarkosic arenites and graywackes. Grain size ranges from fine to coarse sand and the sands vary from poorly to well sorted. Burial depths of the Vedder sands exceed 10,350 ft and the reservoir temperature is 255ºF. The Eocene sands are buried at greater than 12,861 ft and the reservoir temperature is estimated to be around 275°F. These sands are currently at or very near their deepest burial depths. Porosity within the Vedder and Eocene sands is controlled by compaction, dissolution of framework grains, and cementation. Mechanical compaction is evident by long, concavo-convex and sutured grain-to-grain contacts, fractured and broken framework grains and cements, and deformed labile grains. Compaction reduced primary porosity through readjustment of grains, fracturing and subsequent rotation of grain fragments, and deformation of micas and labile grains. Precipitation of cements, including phosphate, clays, calcite, dolomite, K-feldspar, quartz, barite, anhydrite, and pyrite, also reduced porosity at various times during burial. Other diagenetic processes included glauconization of feldspars, quartz, and chert, phosphate replacing feldspars, glauconite, and quartz, replacement of framework silicates by carbonates, alteration of biotite, albitization of feldspars, dissolution of framework grains and carbonates, and dolomite replacing calcite. Dissolution of feldspars, quartz, volcanics, micas, and carbonates created secondary porosity and altered QFRf and QFL ratios. Deeper samples are quartz-rich relative to shallower samples, suggesting feldspar removal through dissolution. Dissolution affected plagioclase more than K-feldspar. Continued compaction reduced both primary and secondary porosity and most likely permeability, while continued dissolution of framework grains and cements maintained an open pore network, thus facilitating the migration and accumulation of hydrocarbons. Pyrite formed after emplacement of hydrocarbons suggesting continuing thermal maturation within the reservoir. Textural relationships of the diagenetic minerals suggest syndepositional formation of glauconite, phosphate, and early pyrite, followed by early precipitation of pore-lining clay coatings and carbonate cements, along with framework-grain fracturing and possibly dissolution. With increasing burial, dissolution of framework grains continued, accompanied by albitization of feldspars, formation of K-feldspar and quartz overgrowths, precipitation of kaolinite and other clays, and possibly precipitation of late carbonate cements. Finally, hydrocarbon migration and formation of late pyrite occurred during late diagenesis.
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