Reservoir Description Breathes New Life Into an Old Fireflood: MOCO T Reservoir, Midway-Sunset Field, Kern County, California

B. R. Hall

The MOCO T reservoir is a Miocene-age ("Stevens equivalent," Monterey Formation) unconsolidated sand reservoir in the Midway-Sunset field, Kern County, California. This reservoir was discovered in 1957 as a deeper pay beneath the Monarch and Webster reservoirs. Due to low prices for heavy oil (14° API), the MOCO T was only partially developed and remained essentially shut-in until initiation of in-situ combustion in 1960. Exploitation of the MOCO T by the combustion process continues today, with cumulative production to date of approximately 14 million bbl of oil.

The MOCO T reservoir is approximately 500 ft thick and lies at an average drill depth of 2,100-2,700 ft. Based on modern core data and sand distribution maps, these sands were probably deposited by channelized turbidity currents that flowed southwest to northwest in this area. Oil is trapped in the MOCO T reservoir where these sand-channel packages intersect a northwest-southeast-trending anticline.

The in-situ combustion process involves crestal air injection through five wells high on the anticline and 30 producing wells on the flanks of the anticline. The effects of in-situ combustion, gravity drainage, and limited natural water drive have resulted in an oil leg that is balanced on the flanks of the structure between the combustion front above and the encroaching water below.

Detailed recorrelation of wireline logs, stratigraphic zonation, and description of individual zones of the MOCO T reservoir in the context of a channelized turbidite system have led to: (1) determination of probable flow paths, vertically and laterally, between injectors and producers by zone, (2) control for workovers to optimize conformance between injection and production intervals, and (3) identification of previously unrecognized and undeveloped reserves.

Integration of this geologic model with an understanding of how the combustion front has advanced through the MOCO T reservoir has led to successful placement of infill wells to produce the reservoir more efficiently and completely.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.