Characteristics of the C Shale and D Shale Reservoirs, Monterey Formation, Elk Hills Field, Kern County, California
S. A. Reid, Jana L. McIntyre, and George S. McJannet
The upper Miocene C Shale and D Shale reservoirs of the Elk Hills Shale Member of the Monterey Formation have cumulative oil and gas production much higher than the originally estimated recovery. These San Joaquin basin reservoirs are the lowest of the Stevens producing zones at Elk Hills and currently produce from a 2800-acre area on the 315 anticline. The C Shale contains lower slope and basin plain deposits of very fine grained, thinly bedded, graded turbidites, pelagic and hemipelagic claystone, and slump deposits. Although all units are oil-bearing, only the lower parts of the graded turbidite intervals have sufficient horizontal permeability to produce oil.
The D Shale consists of chert, claystone, carbonates and slump deposits, also originating in a lower slope to basin plain setting. All D Shale rock types contain oil, but the upper chert interval is the most productive. The chert has high matrix porosity, and due to a complex horizontal and vertical microfracture system, produces at a highly effective rate.
Core samples indicate more oil-in-place is present in the thin, graded C Shale beds and in the porous D Shale chert than is identifiable from conventional electric logs. High gas recovery rates are attributed mostly to this larger volume of associated oil. Gas also enters the reservoirs from the adjacent 26R reservoir through a leaky normal fault. Significant gas volumes also may desorb from immature organic material common in the rock matrix.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California