James A. Peterson
U.S. Geological Survey Department of Geology
University of Montana
Missoula, Mont.
The northern Rocky Mountain-Great Plains area includes nine main petroleum exploration provinces.
These are the Wyoming-Utah-Idaho Thrust Belt, southwestern Wyoming basins, the Big Horn basin, the Wind River basin, the Powder River basin, western Montana, Sweetgrass arch, Central Montana, and the Williston basin-Sioux uplift province (Fig. 1).
More than 2,500 oil and gas fields have been discovered in these provinces with cumulative production as of 1986 of about 8 billion bbl of oil and more than 15 tcf of gas (Fig. 2). Twenty-five giant fields, many of which were discovered early in the century, account for more than half of the cumulative production (Table 1).
Oil and gas production is from carbonate and sandstone reservoirs ranging in age from Cambrian to Tertiary. Organic rich petroleum source rocks are resent in the Ordovician, Devonian, Mississippian, Pennsylvanian, Permian, Cretaceous, and Tertiary stratigraphic sequences.
USGS mean estimates of undiscovered conventional recoverable petroleum resources in the region are about 4.4 billion bbl of oil and 29 tcf of gas.
The Phanerozoic sedimentary cover in the northern Rocky Mountain-Great Plains region ranges in thickness from more than 30,000 ft in some of the southwestern and south central Wyoming basins to less than 5,000 ft in northern Montana (Figs. 3-7).
A relatively complete stratigraphic section is present in the Willison basin and most of the Wyoming basins, but in central and northern Montana two major unconformities, the sub-Upper Devonian and sub-Middle Jurassic, document the erosion or non-deposition of much of the Ordovician, Silurian, Lower Devonian, Pennsylvanian, Permian, Triassic and Lower Jurassic sedimentary record (Fig. 3-7).
The Paleozoic section also thins markedly in south central and southeastern Wyoming, northwestern Nebraska, and southeastern South Dakota along the flank of the transcontinental arch. Above the basal Cambrian clastic section, lower and middle Paleozoic rocks are dominated by shelf carbonates, whereas the upper Paleozoic, Mesozoic, and Tertiary rocks are primarily marine and continental clastics. Thick, lower Tertiary lacustrine sedimentary rocks are present in some of the southwestern and south-central Wyoming basins.
PETROLEUM PROVINCES
WYOMING THRUST BELT
The Wyoming-Utah-Idaho Thrust Belt is a north-south region of eastward thrusting that developed during late Mesozoic and early Tertiary time on the western shelf margin or hingeline of the Rocky Mountain Paleozoic shelf.
Shelf-margin and geosynclinal marine carbonate and clastic beds more than 50,000 ft thick were folded, thrusted, and transported eastward during the Laramide orogeny to come into juxtaposition with organic-rich Cretaceous shales.
Fifteen clastic and carbonate reservoirs of Ordovician through Cretaceous age are productive in structural traps associated with the main thrust sheets.
Clastic reservoirs are Pennsylvanian, Jurassic, and Cretaceous age; carbonate reservoirs are of Ordovician, Devonian, Mississippian, Permian, Triassic, and Jurassic age.
As of mid-1986, about 500 million bbl of oil and 7 tcf of gas had been discovered. Cumulative production totaled 70 million bbl of oil and condensate and 1 tcf of gas. The thrust belt contains four giant fields ranging in size from 115-145 million bbl of oil.
Plays for unfound conventional petroleum in the thrust belt are mainly concerned with continuation of established production plays. The main plays are the Absaroka thrust oil and gas play, the Hogback thrust oil and gas play, the subthrust northern Moxa arch gas play, the Crawford-Meade thrust gas play, and the Cretaceous stratigraphic oil and gas play.
USGS estimates of undiscovered conventional petroleum resources in the thrust belt province range between 210-1,190 million bbl of oil and 6.29-31.31 tcf of gas. Mean values are 580 million bbl oil and 15.81 tcf of gas.
Less than 10% of the undiscovered conventional resources are expected to be in fields containing less than 1 million bbl of oil or 6 bcf of gas (Fig. 8, Table 2).
SOUTHWESTERN AND SOUTH CENTRAL WYOMING BASINS
This region includes the Green River, Great Divide, Washakie, Sand Wash, Hanna, Shirley, and Laramie basins. Also included are the associated Moxa, Wamsutter, and Cherokee Ridge arches, and the Rock Springs and Rawlins uplifts (Fig. 4).
In the Greater Green River basin, Phanerozoic sedimentary rocks are as much as 30,000 thick in the various basin centers (Fig. 3, 4, 7). Most of the production is natural gas from Cretaceous and Tertiary marine and continental sandstone reservoirs (Fig. 2,4,7). Most accumulations are on or adjacent to the Moxa arch, the Rock Springs uplift, Wamsutter arch, and Cherokee Ridge arch. Source rocks are carbonaceous shale, siltstone, and coal beds of Cretaceous and early Tertiary age, and lacustrine beds of early Tertiary age.
The main plays for new reserves in the Greater Wind River basin are structural traps and updip stratigraphic traps associated with expansion of existing producing areas in the Moxa, Rock Springs, Wamsutter, and Cherokee Ridge uplifts, basin margin anticlines, and a subthrust play with structural traps beneath high-angle thrusts on the northeastern and southern flanks of the basin region.
Most of the area east of the Greater Green River basin is included in the structural-stratigraphic platform play, which includes the Hanna, Shirley, and Laramie basins and associated Rawlins and Sweetwater uplifts (Fig. 2). These basins are relatively small in areal extent but contain substantial thicknesses of sedimentary rocks ranging from 8,000-10,000 ft to more than 30,000 ft (Hanna basin).
Reservoirs are mostly Pennsylvanian and Cretaceous sandstones. Production has also been established from Jurassic sandstone reservoirs and carbonate reservoirs of Pennsylvanian, Permian, and Cretaceous age. Three giant oil fields are located in the platform play area, ranging in size from 100-195 million bbl of oil (Table 1).
USGS estimates of undiscovered conventional petroleum resources for this region range from 60-470 million bbl of oil and 1,320-6,760 bcf of gas (Fig. 8, Table 2). Mean values are 210 million bbl of oil and 3.38 tcf of gas. About 30% of the oil and 50% of the gas are expected to occur in accumulations of less than 1 million bbl of oil or 6 bcf of gas.
Not included are gas reserves in low permeability sandstone and siltstone reservoirs.
BIG HORN BASIN
This Wyoming basin is one of the most prolific Rocky Mountain basins.
Eight fields are greater than 100 million bbl of oil in size. The basin has a rather complete section of carbonate and clastic rocks as thick as 25,000 ft. Most fields are on folds or faulted folds on the basin margins (Fig. 2).
Production is mostly from Pennsylvanian Tensleep sandstone, Cretaceous sandstone reservoirs, Ordovician, Devonian, Mississippian, and Permian carbonate reservoirs.
The main plays are aimed at the sub-Absaroka, based on structural traps in Paleozoic reservoirs beneath the lower Tertiary Absaroka Volcanic Supergroup cover on the west flank of the basin; the basin margin subthrust play, with structural traps below high-angle thrusts on the west and east sides of the basin; basin margin anticline play (the original play of the basin, now in the mature stage of exploration); and deep basin structure and stratigraphic play, primarily a gas play, based on the occurrence of deep anticlinal structures in the basin center and isolated or pinchout sandstone reservoirs of Cretaceous and Tertiary age. Isolated carbonate buildups of Permian age also have potential for deep gas reserves.
USGS estimates of undiscovered conventional resources for the Big Horn basin range from about 100-490 million bbl of oil and 1801,590 bcf of gas. Mean values are 250 million bbl of oil and 660 bcf of gas.
About 20% of the oil and 10% of the gas are expected to be in accumulations of less than 1 million bbl of oil or 6 bcf of gas.
WIND RIVER BASIN
This basin has a rather complete section of carbonate and clastic rocks 25,000 ft thick or more.
Most existing oil fields are in structural traps along the southwestern flank of the basin (Fig. 2).
Production is mostly oil from Pennsylvanian Tensleep sandstone, Permian carbonate, Cretaceous, and Tertiary sandstone and Mississippian carbonate reservoirs.
The main plays are aimed at deep basin gas, based on occurrence of deep basin structures with Paleozoic, Mesozoic, and Tertiary reservoirs and isolated and pinchout sandstone reservoirs of Cretaceous and Tertiary age; the Lower Cretaceous Muddy sandstone play, primarily a stratigraphic trap play; the basin-margin subthrust play, based on traps beneath high-angle thrusts on the northeastern, southeastern, and western parts of the basin; and the basin-margin anticline play, the original play of the basin, now in the mature stage.
USGS estimates of undiscovered conventional petroleum resources for the Wind River basin range from 90-370 million bbl of oil and 820-3,550 bcf of gas. Mean values are 200 million bbl of oil and 1,890 bcf of gas.
About 25% of the oil and 30% of the gas are expected to be in accumulations of less than 1 million bbl of oil or 6 bcf of gas.
POWDER RIVER BASIN
Sediments in the Powder River basin thin from a maximum of about 16,000 ft in the northwestern and western parts of the basin to less than 5,000 ft in the southeastern sector.
This is mostly an oil province, with cumulative production of more than 1 billion bbl of oil. The largest fields are structural-stratigraphic traps, but a major share of the oil is in stratigraphic traps on the broad northeast flank of the basin (Figs. 2, 4, 7). Five giant oil fields are present, ranging in size from 108-650 million bbl of oil (Table 1).
Most of the production is from Pennsylvanian, Permian, and Cretaceous sandstone reservoirs.
The main plays are for unfound oil in the Pennsylvanian and Permian Minnelusa formation, a sandstone play and primarily a stratigraphic trap play; deep basin Lower Cretaceous sandstone play in the Dakota and Muddy sandstone reservoirs, the shallow Lower Cretaceous sandstone play, the Upper Cretaceous sandstone play, and basin-margin anticline plays, a maturely explored program.
USGS estimate of undiscovered conventional petroleum resources for the Powder River basin range from about 1.16-3.82 billion bbl of oil and 1.38-4.78 tcf of gas (Fig. 8, Table 2). Mean values are 2.25 billion bbl of oil and 2.76 tcf of gas.
About 25% of the oil and gas are expected to be found in fields of less than 1 million bbl of oil or 6 bcf of gas.
MONTANA OUTSIDE THE
WILLISTON BASIN
WESTERN MONTANA
This region covers the Montana Thrust Belt and adjacent area to the west and the southwestern Montana foreland basins (Fig. 1).
The northern part of the Montana Thrust Belt is a southward extension of the Canadian Rocky Mountain Thrust Belt, where several large oil and natural gas fields are located in the Foothills Belt.
Reservoirs in the western Montana province are mostly porous and fractured Paleozoic carbonates and sandstones. Mesozoic sandstones and Mississippian carbonates in imbricate thrust sheets and folds are potential reservoirs in the eastern part. This province is relatively unexplored.
USGS estimates of undiscovered conventional petroleum resources for the area range from nil to 100 million bbl of oil and 490-9,790 bcf of gas (Fig. 8, Table 2).
SWEETGRASS ARCH
The sedimentary section in this region is generally less than 7,000 ft thick.
Oil and gas are produced from Mississippian marine carbonate reservoirs and Jurassic and Cretaceous marine reservoirs on the Sweetgrass arch and adjacent flanks. Mostly gas is produced from Jurassic and Cretaceous sandstone reservoirs in the rest of the region.
The main plays are aimed at structural and stratigraphic traps with Jurassic and Cretaceous sandstone reservoirs and a continuation of the primarily structural Mississippian and Devonian carbonate-reservoir play in the vicinity of the Sweetgrass arch and near the eastern border of the Northwest Montana Overthrust Belt. Deeper drilling for Devonian carbonate reservoirs is receiving active attention.
USGS estimates of undiscovered conventional petroleum resources range from about 50-180 million bbl of oil and 310-950 bcf of gas (Fig. 8, Table 2). Mean values are 100 million bbl of oil and 570 bcf of gas.
CENTRAL MONTANA
The Central Montana province includes the Crazy Mountain and Bull Mountain foreland basins and associated basement-involved uplifts, (Figs. 17).
Fill is greater than 15,000 ft thick in the Crazy Mountain basin and less than 10,000 ft thick in the Bull Mountain area. Petroleum production is primarily oil from Upper Mississippian, Pennsylvanian, and Cretaceous sandstone reservoirs.
The main plays are aimed at updip stratigraphic traps and stray sandstone bodies embedded in the organic-rich shale sections; structural traps in areas of suitable sandstone porosity, primarily in Upper Mississippian and Pennsylvanian marine and deltaic sandstone lenses. The Crazy Mountain basin is only slightly explored.
USGS estimates for reserves range from 10-60 million bbl of oil and 10-20 bcf of gas (Fig. 8, Table 2). Mean values are 30 million bbl of oil and 10 bcf of gas.
NORTH AND SOUTH DAKOTA
AND EASTERN MONTANA
WILLISTON BASIN-SIOUX UPLIFT
The Williston basin is a classic structural-sedimentary intracratonic basin located on the western shelf of the North American craton.
The relatively complete Phanerozoic sedimentary cover in the basin is greater than 16,000 ft thick in the basin center in northwestern North Dakota and thins uniformly toward the basin margins (Fig. 3, 4, 6).
The basin is primarily an oil province with cumulative production of about 1.2 billion bbl of oil and 1.7 tcf of gas. The large accumulations and about 75% of the total ultimate reserves are on two major anticlinal trends: the Nesson anticline in North Dakota and the Cedar Creek anticline in Montana (Figs. 2, 4. More than 90% of the oil is in carbonate reservoirs, in decreasing order of importance of Mississippian, Late Ordovician, Devonian, and Silurian age.
The main plays are aimed at the sub-Upper Devonian carbonates, Upper Devonian and Mississippian carbonates, deep basin carbonate and sandstone gas play, and supra-Madison and Mesozoic sandstones. All of the plays are strongly stratigraphic in nature and characterized by the occurrence of discontinuous porosity belts in combination with local or regional structure and the distribution of source rocks.
Recent experimentation with horizontal drilling in the Devonian and Mississippian Bakken formation and the Upper Ordovician carbonate beds of the Red River formation has shown promising success and may add significantly to future reserves in the Williston basin. Four giant fields are present in the basin, ranging in size from 100-128 million bbl of oil (Table 1).
The Sioux uplift province has potential for shallow biogenic gas and limited potential for small conventional gas accumulations in Cretaceous sandstone and chalk reservoirs. There is no current production in the area.
USGS estimates for undiscovered resources for the entire region range from about 490-1,150 million bbl of oil and 490-1,070 bcf of gas. Mean values are 780 million bbl of oil and 740 bcf of gas (Fig. 8, Table 2).
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