Abstract

The Coal Gulch-Echo Springs-Standard Draw field complex is one of the largest commercial gas accumulations in the Rocky Mountain region with over 1 TCFG of recoverable reserves. Gas is produced from both "Upper Almond" barrier bar and shoreline sandstones at the top of the Mesaverde Group (Upper Cretaceous) and from underlying "Main Almond" fluvial and marginal marine sandstones.

Recent work suggests that although the bulk of the produced gas in the fields is from the Upper Almond bar interval, simple volumetric calculations can only account for about 50% of the estimated ultimate recovery from this reservoir. This implies the depleting Upper Almond reservoir might be actively recharged by gas leakage from deeper Main Almond sandstones, with contributions from the deeper reservoirs of up to 10 – 30 BCFG per well. This is in stark contrast to typical Main Almond-only producers outside the field area which have mean reserves of less than 1 BCFG and rarely produce more than 2 BCFG per well.

We determined the gas in place for all field wells drilled prior to 1993. The gas in place within the Upper Almond reservoir only was determined by detailed open hole log analysis and volumetric mapping to be 1058 BCFG. Total reserves from all producing intervals (Upper Almond and Main Almond combined) were estimated by decline curve analysis to be 1003 BCFG. The Main Almond lenticular reservoirs contribution to total production was then assumed to be statistically similar to Main Almond-only producers outside the field area, giving an estimated total contribution from Main Almond completions of 96 BCFG. Therefore the recovery factor from the Upper Almond alone is estimated to be (1003–96) / 1058 = 86%. We concluded that field volumetrics do not support a disproportionate contribution of Main Almond gas to the total field production, nor does the volumetric analysis support the active reservoir recharge hypothesis.

Geological setting

The Coal Gulch-Echo Springs-Standard Draw fields are located in the Washakie basin, one of the eastern sub-basins of the Greater Green River basin of southwestern Wyoming and northwestern Colorado (Figure 1). The Washakie basin is bounded on the west by the Rock Springs uplift and on the east by the Rawlins uplift. The Washakie basin is separated from two other sub-basins by minor structural arches: the Wamsutter Arch separates the Washakie basin from the Red Desert (Great Divide) basin to the north and the Cherokee Arch separates it from the Sand Wash basin to the south. Most of the structural movement was during the early Tertiary and post dates deposition of the productive sand trends.

During the Late Cretaceous the western shoreline of the Western Interior seaway moved back and forth across the eastern Green River basin, depositing a thick transgressive-regressive sequence including marine calcareous mudstones, marine shales, offshore marine sandstones, barrier island and shoreline sandstones, coals and non-marine coastal plain sandstones and shales. The study area fields produce gas from a marine shoreline trend at the top of the Mesaverde Group and also from deeper fluvial and near shore marine sandstones, all of which are placed within the Almond Formation. The Upper Almond sandstone is a sequence of barrier-bar, shoreface and tidal delta sandstones from 0 to 40 feet thick.

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