Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Diagenesis Of The Oligocene Vedder Formation, Greeley Oil Field, Southern San Joaquin Basin, California

Abstract

The Vedder Formation 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 exceed 3150 m and the reservoir temperature is ∼125 °C. The sands are bounded below by silts and shales within the lower Vedder Formation and above by deep-marine shales of the Freeman-Jewett Formation. These sands are currently at or very near their deepest burial depths. Porosity within the Vedder sands is controlled by compaction, dissolution of framework grains, and cementation. Mechanical compaction is evident by long 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 and chert, phosphate replacing feldspars, glauconite, and quartz, replacement of framework silicates by carbonates, alteration of biotite, albitization of feldspars, and dissolution of framework grains and carbonates. 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 and phosphate, 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 plagioclase and K-feldspars, formation of K-feldspar and quartz overgrowths, precipitation of kaolinite and other clays, and possibly late carbonate cements. Finally, hydrocarbon migration and formation of pyrite occurred during late of diagenesis.