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Abstract: Geology & Production Implications of Debris Flows in the C & D Shale Reservoirs, Elk Hills Giant Oil Field, California


The C & D shale reservoirs (Monterey Shale equivalent) underlie most of the 31S anticline at the Elk Hills oil field, California. As this giant oil field matures, unconventional reservoirs such as the C & D receive more attention for the purpose of both improving existing oil recovery and identifying additional reserves. To improve the understanding of its internal geometry, the C & D sequence has been cored recently in two wells and a detailed reservoir description study has been completed.

Correlation of existing open-hole logs to the recent cores and to micro-imager logs has led to the discovery of intervals with very chaotic internal architecture that have probably formed as submarine debris flows and slumps. The thickness of these intervals has been mapped and their overall pod-like geometry might be related to incipient growth of the 31S anticline.

These debris flows and slumps are not good reservoirs, but can occupy a significant portion of the interval that normally contains the productive reservoirs. Recognition and understanding of the debris flows and slumps and their overall geometry has implications for production. For example, they frequently have the log character of shales and have been selected as perforation blanks when completing most wells; the effectiveness of these blanks is now in question because of the high incidence of slickensided fractures within the debris flows. Because of the pelagic depositional environment of the shales, intervals that appear to be shales by electric log response were thought to be laterally continuous permeability barriers. These debris flows and slumps are now recognized as not being laterally continuous or extensive but may still act as coal barriers to production and uniform reservoir drainage.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California