Porosity of Unconsolidated Sand, Diatomite, and Fractured Shale Reservoirs, South Belridge and West Cat Canyon Oil Fields, California
Published:January 01, 1987
L. A. Beyer, 1987. "Porosity of Unconsolidated Sand, Diatomite, and Fractured Shale Reservoirs, South Belridge and West Cat Canyon Oil Fields, California", Exploration for Heavy Crude Oil and Natural Bitumen, Richard F. Meyer
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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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Exploration for Heavy Crude Oil and Natural Bitumen
Gross volumes of oil, which must be kept in mind to address the volume/size framework, may be thought of in order from largest to probably smallest volumes as follows: (1) generated; (2) dissipated; (3) degraded/ partially preserved; and (4) trapped and conventionally producible. Basic knowledge of these volumes may be from greatest to least in essentially reverse order.
The 332 largest known accumulations (less than 1% of the total number) account for more than three-quarters of the known 7.6 trillion bbl of oil and heavy oil or tar in more than 40,000 accumulations in the world. About 2.4 trillion bbl of estimated undiscovered conventional oil added to the known volume of 7.6 trillion bbl yields a total of 10 trillion bbl known or reasonably estimated. World-wide cumulative production of about 500 billion bbl of oil accounts for only 5% of the gross.
Oil in place must be estimated for conventional oil fields before comparison with heavy oil and tar accumulations. The size range of accumulations considered in the size distribution of the 332 largest known accumulations is from 0.8 to 1850 billion bbl of oil. The smallest conventional fields in the distribution are about 1 billion bbl because the size cut-off is 0.5 billion bbl of oil recoverable. The size distribution of the 332 largest known accumulations approaches log normal and is overwhelmed by the largest three supergiant tar deposits that hold nearly half of the total 5495 billion bbl.
Globally, the largest three accumulations, all heavy oil or tar, are in South and North America; the two largest conventional oil fields are in the Middle East. Prudhoe Bay and East Texas fields rank 18 and 34, respectively, in descending size order.