DIAGENESIS OF THE OLIGOCENE VEDDER FORMATION, GREELEY OIL FIELD, SOUTHERN SAN JOAQUIN BASIN, CALIFORNIA

The Vedder Formation sands at the Greeley oil field consist of arkosic to subarkosic arenites. Grain size ranges from fine to coarse sand and the sands vary from poorly to well sorted. Burial depths of the Vedder sands exceed 3150 m and the reservoir temperature is ~125C. These sands are bounded below by non-marine sands of the Walker Formation and above by deep-marine shales of the Freeman-Jewett Formation. Porosity within Vedder sands is controlled by compaction, dissolution of the framework grains, and cementation. Mechanical compaction is evident by long and sutured grain-to-grain contacts, fractured and broken framework grains, and deformed labile grains. Compaction reduced primary porosity through readjustment of grains, fracturing and subsequent rotation of grain fragments, deformation of micas and labile grains, and precipitation of cements including quartz and feldspar overgrowths and pore-filling calcite, dolomite, barite, pyrite, phosphate, and clays. Other diagenetic processes included albitization and sericitization of feldspars, glauconization of feldspars and chert, alteration of biotite and glauconite, and dissolution of framework grains. Dissolution of feldspars, quartz, volcanics, and micas created secondary porosity. Preliminary results indicate deeper samples are quartz rich relative to shallower samples, suggesting feldspar removal through dissolution. Plagioclase was affected more than K-feldspar, resulting in higher K-feldspar:plagioclase ratios in deeper samples as well. Continued compaction further reduced the primary porosity, secondary porosity, and most likely permeability. Continued dissolution of the framework grains maintained an open pore network thus facilitating the migration and accumulation of hydrocarbons.