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Abstract

Porosity analysis based on conventional core samples, gamma-gamma logs, and borehole gravity (BHG) surveys is presented for a Pleistocene unconsolidated sand reservoir and for Miocene diatomaceous, porcelaneous, and fractured shale reservoirs of the Monterey Formation in the South Belridge oil field and West Cat Canyon area of the Cat Canyon oil field, California. BHG surveys, the least well-known porosity method, investigate large volumes of the adjacent rock units and provide very accurate measurements of formation density as averages over vertical intervals. BHG and gamma-gamma densities were most discrepant opposite soft, diatomaceous mudstone and hard, naturally fractured, carbonate- and quartz-bearing siliceous rocks, less discrepant opposite unconsolidated sands, and generally in agreement opposite porcelaneous shale and less pervasively fractured quartz-bearing siliceous rocks. These differences resulted from less accurate compensation of gamma-gamma readings for formation damage, mudcake, drillhole enlargement, and rugosity rather than from errors in BHG densities that were very small. Core samples were used to provide grain density needed to calculate porosity from BHG and gamma-gamma densities. Grain densities of core samples from the Monterey Formation range from less than 2.2 to more than 2.7 g/cm3, saturated bulk densities from about 1.4 to more than 2.7 g/cm3, and intergranular porosities from 0 to about 65%. The ranges of BHG and gamma-gamma densities and calculated porosities were nearly as large, reflecting the mineralogic and lithologic diversity (including diagenetic phases of silica) in the Monterey Formation. Quantitative evaluation of fracture porosity by BHG survey in the West Cat Canyon field was inconclusive because insufficient core and well log data were available to determine interval averages of grain density and intergranular porosity with requisite precision in this lithologically diverse and thinly bedded reservoir. Anomalous gravity effects, normally small or insignificant, were equivalent to density corrections of as much as —0.22 g/cm3 for the BHG survey in the South Belridge field and were evaluated with a surface gravity map and the gamma-gamma log.

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