Diagenesis of the Oligocene Vedder Formation, Greeley Oil Field, Southern San Joaquin Basin, California, USA
Nguyen, Diem T.; Horton, Robert A. Jr.; and Kaess, Alyssa B.
The Greeley oil field, located in the southern San Joaquin Basin, has been producing hydrocarbons since 1936; production comes mainly from sandstones in the Vedder Formation and the Stevens sands within the Monterey Formation. Burial depths of the Vedder Formation exceed 3150 m and reservoir temperature is ~125ºC. Vedder Formations sands consist of poorly-to-well sorted fine-to-coarse arkosic arenites. They are bounded below by non-marine sands of Walker Formation and above by deep-marine shales of Freeman-Jewett Formation. Study of outcrops near the eastern margin of the basin suggests that the sands were deposited in a shallow marine environment.
Porosity within the 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 overgrowths and pore-filling calcite, dolomite, barite, and clays. Other diagenetic processes included albitization and sericitization of feldspars, alteration of biotite, 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 shallow samples, suggesting feldspar removal through dissolution. 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.
AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013