We dont usually
think of the Williston Basin as a gas basin. It has generally been under
the radar while most of the Rocky Mountain gas basins have been
leased up. In central areas where associated gas is produced with deep
oil, leasing in the Williston Basin can also be a bit complicated. But
around the margins, extensive lease tracts are readily available and existing
accumulations demonstrate shallow gas potential.
Our tour of the basin is organized around production clusters on three
Specific fields and structures discussed on each of the three margins
are marked by stars and letters on the Williston Basin map. These leads
are also all included in the map and tables that constitute the next set
of leads in the Rocky Mountains.
The northwestern margin of the Williston Basin is a mature shallow gas
province with a long history of production. Initial production was in
the early twentieth century; major production was generally established
by the 1930s; modern expansion started in the 1970s and continues
into the 1990s. The history of production is closely tied to the
huge fields on the southeastern margin of the Alberta Basin, CA, Leads:
Figure 1). This is also true for exploration models.
Exploration strategies have changed substantially. Initially,
buoyancy traps in conventional reservoirs were developed as small sweet
spots. Later, more continuous, commodity-type development involved step-out
drilling from areas of established production. This phase involved mostly
engineering applications with little useful input from geology.
A new exploration model has recently been developed from
geologic studies done by the USGS. Aspects of it are included in the Shallow
Gas Ideas portion of this website. Basically, the gas is thought
to be early generation biogenic gas generated shortly after deposition
of the Cretaceous clastic rocks. The rocks act as both source beds and
as unconventional, tight reservoirs. Gas is trapped within
paleotectonic lineament blocks and has not migrated significantly through
the low permeability rocks. This model needs to be applied in areas away
from current production.
Specific accumulations and leads include old established
production in the area of the Bearpaw Uplift and Bowdoin Dome and new
discoveries and prospects around Poplar Dome.
1. Bearpaw Uplift (BU, Leads:
Figure 1)--Numerous small fields (see 11-19, Table
1, in the Rocky Mountain Basin list) in the vicinity of the uplift
are generally buoyancy traps in conventional reservoirs of the Cretaceous
Eagle Formation. The largest field is Tiger Ridge which has produced more
than 76 BCF (Montana Geological Society, 1985).
2. Bowdoin Dome (BD, Leads:
Figure 1)--Unconventional reservoirs in the Cretaceous Belle Fourche,
Greenhorn, and Carlile Formations cover a large area. More than 700 wells
are spread throughout 600 sq mi; cumulative production is grater than
200 BCF (Rice and others, 1990).
3. Poplar Dome (PD, Leads:
Figure 1)--Prospects are available on the Ft Peck Reservation (Monson,
1995) in several Cretaceous formations. A shallow gas discovery was reported
northeast of the reservation in the spring of 2001, but subsequent development
has been limited.
The southwestern margin of the Williston Basin is an emerging shallow
gas province that has had a flurry of recent activity. Like the northwestern
margin, clastic Cretaceous rocks are both source beds and reservoirs and
the gas has an early generation biogenic origin. The history of development
is also similar. Early production emphasized migrated gas in conventional
reservoirs in buoyancy traps on specific geologic structures; more recently,
in-situ gas in unconventional reservoirs has been targeted with modern
Specific leads tend to cluster in two separate areas: Cedar Creek Anticline
Figure 1) in eastern Montana and western North Dakota and the West
Short Pine Hills (WSPH, Leads:
Figure 1) area in northwestern South Dakota.
Cedar Creek Anticline is the oldest and largest field on
the southwestern Williston Basin margin. Gas was discovered in 1912 and
more than 140 BCF has been produced from hundreds of wells (Clement, 1987).
Early production was mainly from high porosity sandstones in the Judith
River Formation and the Judith River is still a viable economic target.
However, most of the recent development work has involved multi-stage
frac jobs on unconventional reservoirs in the Eagle Formation.
Around the margins of Cedar Creek Anticline there are three small fields
located on minor anticlines: Little Missouri (LM), Gaslight (G), and Plevna
(PL). In the fall of 2001, a coalbed methane play was reported on the
northeastern flank of Cedar Creek Anticline into eastern North Dakota.
West Short Pine Hills (WSPH) Field has produced more than
15 BCF from conventional reservoirs in the Eagle-equivalent Shannon Sandstone.
The field is located on an asymmetric anticline draped over a basement
fault; the structural motif is thus very similar to Cedar Creek Anticline.
There may be a significant component of fracture control in West Short
Pine Hills Field (Shurr, 2000), just as documented in the Little Missouri
Field (LM) near Cedar Creek (Shurr, 2001). Cady Creek Field is located
east of West Short Pine Hills Field on a small anticline. Reservoirs are
also in the Shannon Sandstone, but have lower porosity and permeability.
For twenty years after the initial discoveries, there was not much action
in the area of West Short Pine Hills. However, during 2000 and 2001 leasing
has increased dramatically and half a dozen shallow gas tests were drilled.
The geology does explain why there were shows, but no commercial production
from these tests. In particular, the exploration model developed on the
northwestern margin needs to be applied on the southwestern margin. This
means that early generation biogenic gas must be targeted on paleotectonic
lineament blocks and not just in buoyancy traps on present day anticlines.
The approach should be used in both the West Short Pine Hills and Cedar
GeoShurr prospects on the southwestern margin of the Williston
Basin are based on both the new and the old concepts. Descriptions are
found in the part of this website that focuses on prospects.
The eastern margin of the Williston Basin currently has no shallow gas
production and exploration activity is very limited. However, in the early
1900s, there was extensive production and local consumption. A number
of towns in North Dakota and South Dakota had municipal gas systems to
provide heat and light.
There are three areas that had extensive historic production: Souris River
Figure 1) area in north central North Dakota, LaMoure County (LC,
Figure 1) area in southeastern North Dakota, and along the Missouri
River near Pierre (P, Leads:
Figure 1) in central South Dakota.
The Souris River (SR) area has numerous historic shows,
as well as municipal production. The patterns of gas shows correspond
with lineament zones mapped on satellite images and suggest that fractures
are important (Shurr, 2001). Shallow gas was produced in areas where the
Cretaceous Fox Hills and Hell Creek and Tertiary coals subcrop beneath
glacial drift. The gas is found in an artesian ground water flow system
and may be late generation biogenic gas. Although there is currently no
shallow gas exploration, operators drilling for deeper oil have started
putting on gas detectors at the grass roots. And, shows of shallow gas
have been reported.
LaMoure County (LC) also has historic gas production from
an artesian flow system at shallow depths beneath glacial drift. However,
the reservoir rocks are in the Cretaceous Dakota Sandstone. The area of
historic shows and production covers about 25 townships and is elongate
parallel with linear features in a lineament zone. Several recent prospects
have been put together south and west of LaMoure County, but exploration
activity in the general area is very limited.
Along the Missouri River at Pierre (P) South Dakota there
is an area of several thousand square miles that has historic shows of
shallow gas. The capital city of Pierre used gas produced from wells within
the city limits for more than three decades in the early twentieth century.
The gas was produced from the artesian ground water system in the Dakota
Sandstone. Lineament blocks mapped on satellite images appear to correspond
with patterns of gas shows and local consumption (Shurr, 2000). Exploration
programs in the 1970s targeted objectives in the Niobrara Formation,
where a few shows are recorded, as well as in the Dakota Sandstone. Since
then, there has been virtually no exploration activity in the area.
GeoShurr prospects on the eastern margin of the Williston
Basin combine new exploration concepts and unique data sets and unique
data sets in the areas of historic shows. Descriptions of the prospects
available are found in the prospect portion
of this website.