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Abstract: Turbidite Reservoir Compartmentalization: An Example from the 2 Billion-Barrel Midway-Sunset Field, San Joaquin Basin, California

STEWARD, DANIEL, Chevron North American Exploration & Production, Bakersfield, CA

Despite well spacing approaching 1/3 acre, the 550 foot-thick Monarch turbidite sandstone reservoir at Midway-sunset field, San Joaquin basin, California is inadequately drained, even after 25 years of enhanced oil recovery. Foot-scale reservoir compartments prove to be the antithesis of "tank-like" drainage, assumed during early development of the reservoir. As such, the Monarch provides a useful analog for development of deepwater clastic reservoirs elsewhere.

Open-hole logs from 521 Monarch wells drilled since 1973 allows an unprecedented look at reservoir compartmentalization. Diatomaceous and terrigenous shales result in internal flow barriers, sometimes less than a foot thick. Typically, these shales have only a few millidarcies permeability, compared to multi-darcy permeability of gravel-rich sandstones beds of the Monarch. Previously, this turbidite channel complex was treated as a collection of massive sands. However, the reservoir more closely approximates an amalgamation of hundreds of thin (3-20 feet-thick), fine- to coarse-grained sandstones, which filled bathymetric lows and exhibit small-scale basal erosion. In addition to this stratigraphic heterogeneity (particularly along-strike and vertically), the internal reservoir geometries include onlap/downlap, erosional truncation ("pseudo toplap") within synclinal and homoclinal structural settings, and downdip growth of section.

Understanding reservoir complexity generally presents new opportunities for recovering "by-passed oil". Recognizing the range of possible complexities in deep-water reservoirs, such as the Monarch, helps mitigate the risk of well-planning and ease the challenge of long-range field development.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California