Abstract

A petrophysical study of the Frontier Formation in the Whiskey Buttes Field was performed as part of the GRI project "Emerging Resources in the Greater Green River Basin". Tight gas sands in general, and the Frontier Formation in particular, are difficult to evaluate and predicting long term well behavior is a problem. This study provided a detailed petrophysical characterization of the Frontier Formation which allowed us to compare reservoir characteristics to well performance.

Thirty-five representative wells were selected for analysis. The wells were chosen to provide full geographic coverage of the field, to cover the entire range of porosity and permeability and to span the range of production from dry holes to 7 BCF wells. Seven of the wells had routine core analysis available and these were used to calibrate the petrophysical model. A complete shaly sand analysis was performed. Two different pay cases were calculated, generating average effective porosity, permeability, and water saturations for each stratigraphic zone. Also the net feet of hydrocarbon pay volume [h* *(1-Sw)] was calculated for each zone in each well.

These pay parameters were then compared with estimated ultimate recovery from decline curves to evaluate which factors could be used to predict well performance. While several trends could be discerned, no one or two factors alone can be used to predict well performance. Log calculated permeabilities were the best predictor, although not a highly reliable one. The fluvial section of the Frontier appears to contribute more to production than the marine zones. Most of the wells drain less than 160 acres. The major problem is that many of the factors affecting production cannot be quantified, including the effectiveness of the hydraulic fracture treatments, presence of natural fractures, and the effective drainage area for each well.

Setting

The Whiskey Buttes Field is located on the Moxa Arch, in Sweetwater and Lincoln Counties, Wyoming (Figure 1). The field was discovered by Amoco in 1975 through a combination of seismic and subsurface geology. The discovery well, the Whiskey Butte Unit #1 (NW/4 SW/4 S24-T22N-R111W), was completed in the Second Frontier Formation with an initial potential of 1995 MCFGPD and 94 BCPD. As of 1992, the field wide ultimate recovery for the Frontier Formation alone was estimated to be 247.3 BCFG.

Although some of the production is from the Dakota Formation (Lower Cretaceous), the majority of the completions and production are from the Second Frontier Formation (Upper Cretaceous). The Second Frontier has been further subdivided into several locally mappable benches; all but the top bench are marine deposits composed of shoreface, coastal plain, and shoreline facies. The first (top) bench of the Second Frontier is a continental deposit consisting of fluvial valley fill and alluvial plain facies. Nearly all of the wells at Whiskey Buttes were perforated in both the first and second benches, including the fluvial sandstones and the uppermost part of the marine section, but several wells were also completed in the deeper third and fourth benches. We used the stratigraphic nomenclature and correlations proposed by Forster and others as the stratigraphic framework for this study.

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