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.
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
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.
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
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