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Diagenesis And Reservoir Quality Of The Oligocene Vedder Sandstone, Rio Bravo Oil Field, California


The Rio Bravo oil field is located in the southern San Joaquin basin, fifteen miles northwest of Bakersfield. Production from this field comes mainly from sandstones in the Vedder Formation and the Stevens sands that lie within the Monterey Formation. The Vedder Sandstone is the zone of importance and is about 1,250 feet (380m) in thickness. The thin (<100ft, 30m) Miocene Rio Bravo sandstone, which unconformably overlies the Vedder, is included in the main Vedder reservoir. The Vedder reservoir sandstones are bounded below by non-marine sands of the Walker Formation and above by deep-marine shales of the Freeman-Jewett Formation. Burial depths range from approximately 10,750 feet (3,415m) to 12,450 feet (3,800m), with reservoir temperature at approximately 120 degrees C. Samples were obtained from eight wells housed at the California Well Sample Repository. The mineralogy and lithology of Oligocene sandstones of the Rio Bravo oil field were examined using a petrographic microscope and a scanning electron microscope equipped with and energy dispersive x-ray spectrometer (SEM-EDS) and a cathodo-luminescence imaging system (SEM-CL). The Vedder reservoir sandstones are medium to fine-grain subangular to subround very poorly to well sorted arkosic arenites and wackes. Detrital minerals of the Vedder reservoir include quartz, potassium feldspar (orthoclase, microcline, perthite), plagioclase, biotite, muscovite, zircon, hornblende, and rutile. Rock fragments include volcanic grains, granitic microphanerites, chert, and shale clasts. The diagenetic features affecting reservoir quality of the Vedder Sandstones are similar among wells. Authigenic minerals include albite, clays, calcite, dolomite, quartz, pyrite, galuconite, and phosphate. Albitization occurs extensively along fractures in plagioclase and K-feldspar grains. Plagioclase shows varying degrees of alteration to clay or sericite. Biotite has been altered to chlorite and pyrite. Cements include clays (kaolinite, chlorite, and illite and/ or mixed-layer illite/smectite or illite/chlorite), carbonates (calcite and dolomite), and pyrite. Kaolinite occurs as pore-filling cement and is commonly associated with feldspar dissolution. Calcite cement occurs within some through going fractures. Calcite has partially to completely replaced framework-grains. Frambroidal pyrite is present in pores containing clays and hydrocarbons. Authigenic quartz developed mostly as syntaxial overgrowths on detrital quartz grains. Porosity within the Vedder reservoir sandstones is controlled mainly by compaction and dissolution of framework-grains. Compaction decreased porosity through ductile grain deformation of shale clasts and micas, which commonly were squeezed into adjacent pores to form pseudomatrix. Rotation and slippage of grains and fracturing of brittle grains is also widespread. Dissolution of framework grains created oversized and elongate pores, with the result that secondary intergranular porosity contributes significantly to overall reservoir quality.