Michael L. Hendricks¹

(1) Hendricks and Associates Inc, Englewood, CO

ABSTRACT: Upper Cretaceous Lewis Formation Toe-of-Slope Sandstone Reservoirs, Hay Reservoir Field, Sweetwater County, Wyoming

Significant natural gas reserves occur within Lewis Formation toe-of-slope sandstone reservoirs in Hay Reservoir Field, Sweetwater County, Wyoming. These sandstones were deposited in submarine fan environments that were associated with the final regression of the Upper Cretaceous sea. Reservoirs in the field are massive and parallel laminated Bouma A and B type deposits, and are principally sublithic and subfeldspathic fine-grained arenites. Secondary porosity is essential for reservoir performance, and porosity developed by the leaching of unstable grains and chert rock fragments. This diagenesis is associated with a basin-centered gas accumulation in the northern portion of the Great Divide Basin. In Hay Reservoir Field, the best reservoirs occur at, or near, the updip pinchout of porous sandstone into impermeable siltstone and shale. Regional evaluation of Lewis reservoirs indicates that this type of structural trapping and facies change is essential for high production rates and reserves.

The Lewis interval at Hay Reservoir Field is the oldest of a series of southward prograding clinoforms. Regional correlations show that Lewis sandstones are also present within younger clinoform cycles in other portions of the Great Divide, Washakie, and Sand Wash Basins, Wyoming and Colorado. Oil and gas production occurs within the same depositional and lithofacies as in Hay Reservoir.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado