--> --> ABSTRACT: Reservoir-Sandstone Paradigms, Paleocene Fort Union Formation, Wind River Basin, Wyoming, by Romeo M. Flores, C. Willaim Keighin, William R. Keefer; #91002 (1990).

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ABSTRACT: Reservoir-Sandstone Paradigms, Paleocene Fort Union Formation, Wind River Basin, Wyoming

Romeo M. Flores, C. Willaim Keighin, William R. Keefer

The Paleocene Fort Union Formation has yielded at least 2.5 million bbl of oil and condensate and 223 bcf of gas from several fields in the Wind River basin, Wyoming. The Fort Union, which includes the Waltman Shale, contains conglomerate, sandstone, siltstone, mudstone, limestone, carbonaceous shale, and coal beds. Studies of cores from the Fuller Reservoir field and of outcrops at the Castle Garden and Hells Half Acre areas suggest that reservoir sandstones formed in fluvial, deltaic, and lacustrine systems.

Fluvial reservoirs are medium- to coarse-grained, trough-crossbedded and convoluted channel sandstones, and fine- to medium-grained, rippled crevasse-splay sandstones. Deltaic reservoirs are mediumgrained, trough-crossbedded, channel sandstones incised into coarsening-upward, trough-crossbedded, rippled, bioturbated deltafront sandstones. Lacustrine reservoirs are very fine to coarse-grained, graded, bioturbated, rippled, slumped offshore sandstones.

Fluvial-sandstone reservoirs are elongate to tabular, and deltaic- and lacustrine-sandstone reservoirs are prismatic. The elongate fluvial reservoirs are mainly randomly organized, composite bodies that are compartmentalized by irregular erosional surfaces marked by discontinuous conglomerate and shale units. A few elongate fluvial reservoir sandstones are compartmentalized only by individual internal structures (e.g., trough crossbeds and convoluted beds). The tabular fluvial and prismatic deltaic and lacustrine reservoir sandstones are primarily homogeneous, multilayered bodies compartmentalized by continuous shale and siltstone units. Fluvial reservoir sandstones are interconnected, forming wing-shaped bodies perpendicular to depositional dip, in contrast to the deltaic-lacustrine ffshore reservoir sandstones, which are wedge-shaped along depositional dip. Fluvial reservoir sandstones are bounded by gas-prone source rock facies and the deltaic-lacustrine offshore reservoir sandstones by oil-prone source rock facies.

Recognition of sedimentological characteristics in outcrops of potential Fort Union reservoir rocks provides insight into reservoir heterogeneity (e.g., shape, internal architecture, sedimentary structures, and textural framework) in the subsurface, which may significantly affect economic development and production.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990