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Rocky Mountain Basins

More than seventy shallow gas accumulations have been identified (Shurr, 2001) on basin margins in the Rocky Mountains (Leads: Figure 2). All of these are in Cretaceous rocks; the formations, field names, and references are listed on Table 1 and Table 2 . The first nineteen leads are around the Williston Basin and have already been described..

The basins that host the remaining leads fall into two broad subdivisions. The “Eastern Basins” include the Alberta, Powder River, and Denver Basins. These three basins are asymmetric, yoked basins; the steep western margin is bounded by faulted uplifts and the eastern margin slopes gently up onto the craton. The “Western Basins” are the classic fault-bounded basins of the Rocky Mountain Foreland. These include the Crazy Mountain Basin on the north, down to the Uinta and Piceance Basins on the south.

Basin margin accumulations in the Rocky Mountains have distinctive characteristics (Shurr, 2001). In general, they are <2000 ft deep, are under pressured, and are dominantly biogenic gas. Large, continuous accumulations in unconventional reservoirs cover >1000 sq mi and are located on regional domes and arches. Smaller, sweet spots in conventional reservoirs cover an average of about 15 sq mi and are located on local geologic structures. Both categories have average reserves on the order of 1 to 2 BCF/sq mi. Foreland structures constitute a third, more limited category. Gas accumulations on foreland structures are slightly deeper, are not under pressured, are smaller, and usually include thermogenic gas in the conventional reservoirs.

EASTERN BASINS

The Alberta, Powder River, and Denver Basins are all located in the transition area between the craton and thrusted mountain fronts. All three basins may have small fields with associated gas on foreland structures located on the steep, western basin margins. However, this discussion emphasizes the larger accumulations that characterize the gently sloping, eastern basin margins.

The Alberta Basin has an extremely important shallow gas accumulation on its southeastern margin. The Southeast Alberta System (20) and the extension into Southwest Saskatchewan (21) cover more than 6000 sq mi and have reserves estimated at more than 10 TCF. The gas is early generation biogenic and the unconventional reservoirs are found in at least four separate Cretaceous formations. The rocks are interbedded fine sandstone, siltstone, and shale with local build-ups of clean sandstone.

These fields were among the first shallow gas accumulations to be studied using modern geologic ideas and technology. It has been extensively employed as an analog for continuous, commodity-type shallow gas accumulations. In fact, the model for early generation biogenic gas being trapped on paleotectonic lineament blocks is based on this area, as well as on the northwestern margin of the Williston Basin.

The Powder River Basinhas five small fields (22-26) around the northern and eastern margin. Most are shut in or abandoned, but Liscom Creek Field (23) has had modern technology installed and is producing. Reservoirs are fairly conventional and are distributed through the Cretaceous section from the Muddy-Newcastle Sandstone up to the Shannon. The fields are confined to small anticlines and represent sweet spots within an area where a continuous, unconventional type accumulation may exist.

Coalbed methane production from Tertiary rocks has recently undergone huge development on the eastern margin of the Powder River Basin. The gas is probably late generation biogenic and is related to ground water flow off the adjacent Black Hills.

The Denver Basin has more than thirty shallow gas fields in chalks of the Niobrara Formation on the eastern margin. Seven of these fields (41-47) account for 75% of the production (Hemborg, 1993). Fields are sweet spots located on faulted anticlines and natural fracturing is important in the high porosity, low permeability chalk reservoirs. During the past six months, there has been a dramatic increase in activity--after a lull of about twenty years. The play extends eastward into Kansas, where the gas may have a late generation component. In Colorado, gas is early generation biogenic on the shallow basin margin; deeper in the basin it is more thermogenic and may be associated with oil production.

WESTERN BASINS


More than half a dozen basins make up the belt of western basins located on the faulted Rocky Mountain Foreland. In general, these basins are smaller and more closely related to the bounding uplifts when compared with the eastern basins. All of the shallow gas accumulations are in clastic Cretaceous rocks. Three basins--the Uinta, Piceance, and Washakie Basins--have most of the larger basin margin gas fields. The remaining five basins have mainly small fields on foreland structures.

Basin margin fields are most numerous in the Uinta and Piceance Basins and are especially important along the Douglas Arch that separates the two basins. More than half a dozen fields (57-63) are located on the arch and they have attributes similar to gas fields around the Bearpaw Uplift on the northwestern margin of the Williston Basin. The Washakie Basin has three basin margin fields, but also has one field on a foreland structure.

Foreland structures host gas fields in five basins: Crazy Mountain, Big Horn, Wind River, Great Divide, and Green River Basins. Essentially all of the fields have anticline traps and most have conventional reservoir rocks.




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