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Diagenesis of the Point of Rocks Sandstone (Middle to Late Eocene) , McKittrick Oil Field, California

Brian L. Taylor and Robert A. Horton
Department of Geology, California State University Bakersfield, 9001 Stockdale Hwy., Bakersfield, CA 93311

The Point of Rocks Sandstone consists of arkosic arenites interbedded with minor shales that were deposited in turbidite depositional systems. This study investigated diagenesis and porosity development within the sandstones. Whole core samples were obtained from four wells in the McKittrick oil field at depths between 9200 and 10850 ft. These were thin sectioned and examined using optical petrography combined with SEM-EDS. Diagenesis included mechanical and chemical compaction, alteration of framework grains, precipitation of authigenic minerals, and dissolution of grains and cement. Authigenic minerals generally constitute about 14% of the sandstone and include clays, albite, laumontite, calcite, and quartz. Detrital plagioclase has been mostly altered to albite. Compaction has had the greatest influence on reservoir quality. Porosity (ranging from 3 to 17% of the rock volume) and permeability were considerably decreased due to grain rearrangement, fracturing of brittle grains, and deformation of ductile grains to form pseudomatrix that flowed into adjacent pore spaces. Thin-section porosity varies between 3 and 17% by volume. Most of the present porosity is secondary and the composition of the sands has been altered due to leaching of feldspars, with a notable decrease in plagioclase feldspars with increasing depth. Feldspar dissolution was accompanied by formation of clay and laumontite cements, but mass balance calculations indicate a net export of aluminum out of the system. Porosity and permeability are largely controlled by variations in cement abundance although locally tectonic-deformation bands are a major cause of porosity and permeability reduction.

 

AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California