REID, S. ANTHONY, and JANA L. McINTYRE
Occidental of Elk Hills, Inc., Tupman, CA
Abstract: Preservation of Porosity in Quartz-Phase Porcelanite Reservoirs, Monterey Formation, Elk Hills Field, California
Production from Monterey Formation porcelanite at the Elk Hills Field in California's San Joaquin basin occurs primarily from the 31S D and 29R AB reservoirs. Productive intervals are clearly quartz phase, but characteristics differ from typical chert reservoirs of the coastal California Monterey Formation in that matrix porosity is more similar to that of the opal CT phase, petroleum storage is mostly in the matrix, and natural fracture patterns are dominantly small scale.
Although interbedded with siliceous shale, sandstone and dolomite, most of the porous reservoir rock is laminated porcelanite. Porosity averages 25 percent, and is evenly distributed throughout the porcelanite as extremely small pores ranging in size from 1 to 10 microns. Matrix permeability averages 0.8 md, but flow of oil and gas is enhanced through fractures parallel to, 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. Significant compaction does not occur with alternation to quartz phase at Elk Hills. Rather, precipitating quartz replaces the previous opal CT structural framework as a pseudomorph. Excess formation waters can not precipitate quartz in the petroleumfilled porosity, and is expelled from the reservoir via the fracture system.
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California