Abstract

The Hay Reservoir Field is located in the Great Divide Basin of southwestern Wyoming. Since its discovery in 1976, 28 wells have been completed from overpressured sandstones within the Upper Cretaceous Lewis Shale. The productive sandstones are very fine- to fine-grained, arkosic arenites with porosities of 5-12% and permeabilities of .01-.6 md. Hydraulic fracturing is required to obtain commercially productive rates. Initial potential flows averaged 2 MMCFGPD and 50 BC at depths of 9,200-10,600 ft (2,804-3,231 m). Reserves of 85-90 BCF have been estimated for the field with reserves per well averaging 4 BCF on 640 acre spacing.

Within the Hay Reservoir area, the Lewis Shale is approximately 1,400 ft (427 m) thick and contains eight sandstones which develop locally in its lower portion. These sandstones are located at the tow of clastic wedges which thicken northeastward. Individual sandstones have a lobate geometry and laterally shift and vertically stack along a definable depositional trend. This geometry, depositional style, observed sedimentary features in cores, and vertical and lateral facies relationships indicate these sandstones are submarine fans deposited by sediment gravity flow processes. These deposits are related to delta front instabilities in a narrow shelf setting. Where these sandstones pinchout in an updip direction, as at Hay Reservoir Field, petroleum traps are formed.

Lewis sandstones at Hay Reservoir exhibit multiple phases of burial diagenesis followed by porosity enhancement by partial dissolution of porefilling cements and more labile feldspar and lithic grains. Porosity preservation appears to be enhanced in the structurally highest position within the sandstones.

A cross plot of productivity factor, which includes parameters of moveable gas volume and permeability, and estimated ultimate recovery for the Lewis "F" sandstone shows four groupings of wells that have a strong correlation of productive character to a submarine fan depositional setting. From highest quality to lowest these include 1) channel axis, 2) mid to distal fan 3) channel margins and 4) any fan position located over prexisting depositional topography.

Recognition of the deep-marine aspect of the Lewis Sandstone reservoirs documents an under-emphasized depositional setting within the Cretaceous Western Interior Seaway. A knowledge of the reservoir and production characteristics of the Lewis at hay Reservior Field should provide data useful in subsequent exploration and development programs for the Lewis in other portions of the Great Divide and Washakie Basins.