Reservoir Description and Production History; Bell Creek Field, Muddy Sandstone, Barrier Island, and Valley Fill Deposits

Roderick W. Tillman, M. Szpakiewicz, M. M. Honarpour, S. R. Jackson

Improved sedimentologic and stratigraphic reservoir descriptions of barrier-island sandstones and genetically related deposits allow better reservoir management. Few published geologic models of ancient barriers contain sufficient detail to predict the distribution of heterogeneities on a variety of scales. The study described involves a 4-mi² area of Bell Creek production Unit A in which 15 cores were described and interpreted and 70 wells were utilized in constructing detailed cross sections and maps.

Production in Unit A is primarily from several stacked shallowing-upward barrier-island sandstones. Within the barrier, facies recognized are foreshore, upper shoreface, lower shoreface, transition, washover, and perhaps a backshore. Two periods of post-barrier erosion and shale and sandstone valley fill are related to two falls in sea level. Earlier valley incisions are broad and relatively shallow and commonly involve reworking of the top of the barrier. Younger valley fill deposits along the west side of Unit A are 30 ft thick where erosion has cut completely through the barrier.

A wide variety of scales of heterogeneities occur within the field: large-scale heterogeneities (facies changes, valley fills, sand-shale boundaries, and faults), moderate-scale heterogeneities (clayey beds, cemented zones, e.g., clay and calcite), and high-permeability channels (perhaps fractures); small scale-diagenetic clay; compaction effects; leaching of minerals; and cementation.

The production history over the 20-year life of the field indicates the effect of various heterogeneities on oil production. The secondary oil production history indicates that as additional fluids were injected, areas of peak production within the field migrated irregularly to the east (updip). Mapping of changes of the percentage of water produced in individual wells over the waterflood history indicates that local heterogeneities within the barrier cause significant local production/injection anomalies.

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