Abstract

Oil and gas production from Monterey Formation porcelanite at the Elk Hills field in California's San Joaquin basin occurs from intervals that have quartz-phase mineralogy. However, characteristics differ from chert and porcelanite reservoirs of the coastal California Monterey Formation in that matrix porosity is more typical of the opal-CT phase, petroleum storage is mostly in the matrix, and natural fracture patterns are dominantly small scale. Several Elk Hills reservoirs located on two large anticlines produce from porcelanite. The 29R AB and 31S D are the most productive porcelanite reservoirs, each having cumulative oil production of about 40 million bbl. Although interbedded with siliceous shale, sandstone, and dolomite, most of the porous reservoir rock is laminated porcelanite. Porosity averages between 20 and 25% and is evenly distributed throughout the porcelanite as extremely small pores ranging in size from 1 to 10 μm. Matrix permeability averages 0.8 md, but flow of oil and gas is enhanced through fractures parallel with and perpendicular to bedding. Higher than anticipated porosity may be in part due to migration of hydrocarbons into the porcelanite reservoirs while still in opal-CT-phase mineralogy. The dissolution of opal-CT and precipitation of quartz occurs in place, and the resulting quartz-phase mineral structure mimicks the porous opal-CT framework.

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