Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Preliminary Study Of Petrology And Porosity Development Of The Oligocene Vedder Sandstones Of The Rio Bravo Oil Field, California


The Rio Bravo oil field is located about 15 miles north-west of Bakersfield, California. The zone of importance is the Vedder Sandstone, which is about 1,250 feet (380m) in thickness. The Miocene Rio Bravo sandstone unconformibly overlies three important oil producing zones within the Vedder: Main Vedder, Osborn, and Helbing. Burial depths range from approximately 10,750 feet (3,415m) to 12,450 feet (3,800m), with reservoir temperature at 120∞C. The mineralogy and lithology of Oligocene sandstones of the Rio Bravo oil field were examined using petrography and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS). The Vedder Sandstones are medium to fine-grain subangular to subround sand that are moderately well sorted. Composition is arkosic arenite with an average composition of framework grains of approximately: 40-55% quartz, 25-40% feldspar; and 5 to 15% rock fragments. The diagenetic features affecting the reservoir quality of the Vedder Sandstones are similar among wells. The destructive features that affect reservoir quality include compaction, albitization of feldspars along fractures within detrital grains, and calcite present as a pore-filling cement as well as a replacement of plagioclase grains. Other diagenetic features that have reduced reservoir quality include authigenic clays, probably including kaolinite and smectite, and quartz overgrowths. The porosity of the Vedder Sandstones consists mainly of remaining primary intergranular pores but also includes a significant amount of secondary porosity. The secondary porosity was mostly created by dissolution of feldspar grains. Fractures within the Vedder Sandstones contain some secondary mineralization, specifically albite occurring along fractures within detrital feldspar grains.