Brief introduction to the geology, structure and oil and gas resources of Bighorn Basin, Wyoming

1. BIGHORN BASIN,
WYOMING
Oil and Gas- an appraisal of Oregon Basin Field
ABSTRACT
The Oregon Basin field, comprising
northern and southern domes, is the
most prolific oil and gas producer in the
Bighorn basin, Wyoming. Oil is produced
from Tensleep sandstone, Embar and
Madison Formations. Gas is mostly
contributed by Chugwater, Embar and
Tensleep sandstone. Though production
of oil in the field has decreased since
1978, production has been around
2,000,000 barrels per year for the last 7
to 8 years and is likely to continue for
some more years. Gas production has
varied over the years reaching a peak in
1998 and has since declined. The
northern dome produced more oil
whereas, the southern dome produced
more gas. The source rock for these
hydrocarbons is Permian Phosphoria
Formation and most likely migrated
during Late Cretaceous to Early Eocene
Laramide orogeny and accumulated in
structural and stratigraphic traps. The
primary reservoir rocks in the Oregon
basin are Tensleep sandstone, Embar,
Madison and Chugwater Formations.
Dr. Venkataramana Pillarisetty

2. Wyoming
Wyoming, the 10th largest state by area (97,914 sq mi) in US, is a major coal (Powder River Basin),
coal bed methane, oil and natural gas producer. It is the 8th
largest producer of oil and natural gas in
US (https://www.eia.gov/state/rankings/#/series/47).
Choropleth Map showing Oil production in counties of Wyoming state
Choropleth Map showing Gas production in counties of Wyoming state
Within the state, Campbell, Park followed by Washakie and Sweetwater counties produce maximum
oil and Sublette and Sweetwater counties produce maximum natural gas.

3. Bighorn Basin, Wyoming
The Bighorn basin in north-central Wyoming and south-central Montana is an intermontane basin of
Rocky Mountain foreland. The basin is approximately 193 km long and 145 km wide and covers an
area of about 13,200 sq. mi. The Bighorn basin covering parts of Park, Big Horn, Hot Springs and
Washakie counties has been a major oil and gas producer in Wyoming. The region is generally flat and
surrounded by mountains on its eastern, western and southern borders. Towards north the basin
merges with the plains in Montana.
Oil and Gas wells in Bighorn Basin, Wyoming

4. Geology of Bighorn basin
The Bighorn basin is a structural basin comprising sedimentary rocks ranging in age from Cambrian
(541 - 485.5 mya) to Miocene (23.03 - 5.33 mya) and thickness of over 25,000 feet (7620 m). The
present structural configuration of the basin was the result of the Late Cretaceous to Early Eocene
Laramide orogeny (Blackstone, 1963). The folding and faulting that formed the present oil-producing
anticlines in the Bighorn Basin occurred during pulses of compressional stress, mainly oriented
northeast-southwest (http://www.wsgs.wyo.gov/energy/oil-gas-basins).
The Bighorn basin is primarily an oil-producing basin. Oil was first discovered here in 1904. The Garland
and Greybull fields were discovered in 1906 and 1907 respectively, both geological structures and
stratigraphy play an important role in localisation of oil in Bighorn basin (Fox and Dolton.1996).
Structural plays include basin margin sub-thrusts, basin margin anticlines, deep basin structures, and
sub-Absaroka-volcanics. Principal stratigraphic plays include Phosphoria pinchout (up-dip facies
change) and Tensleep paleogeography (dune fields versus interdune regions) (Fox and Dolton, 1996).
Significant potential plays include basin center/deep gas and coalbed natural gas
(http://www.wsgs.wyo.gov/energy/oil-gas-basins).
Although most of the basin's production comes from anticlinal or other structural traps, Lawson and
Smith (1966) suggested that many of the structurally-controlled traps are influenced by stratigraphic
effects, including intraformational variations in permeability and, as in the Bonanza and Nowood
fields, incised channels in the Tensleep surface that were later filled with impervious Goose Egg
sediments. Later Laramide folding may have been superimposed on or near these primary traps. Pure
stratigraphic traps are also productive within the Bighorn Basin. The largest of these is the Cottonwood
Creek field in the southeast corner of the basin, a trap resulting Rattlesnake Mountain from an
eastward, updip facies change from Phosphoria carbonate to the impermeable red shale and
anhydrite facies of the Goose Egg Formation (http://www.wsgs.wyo.gov/energy/oil-gas-basins).
The source of essentially all the oil and gas found in Palaeozoic reservoirs in the basin is the dark,
phosphatic, fine-grained, marine facies of the Phosphoria Formation (Stone, 1967). Primary migration
began immediately after deposition of Triassic sediments and was completed by Early Jurassic time.
Hydrocarbons accumulated in regional stratigraphic traps created by up-dip facies changes, pinch-
outs, or truncation of the reservoir rocks in the Phosphoria, and irregular truncation of thick Tensleep
Sandstone beds prior to the deposition of the impervious Phosphoria/Goose Egg Formation. This
situation is especially prevalent east of the area covered by marine carbonate facies of the Phosphoria
Formation (Stone, 1967). Oil and gas in some of these stratigraphic traps were later released by
fracturing and faulting associated with Laramide folding. During the Laramide orogeny, these
hydrocarbons moved into older Palaeozoic reservoir rocks and older structures where they were
trapped in common pools. The occurrence of a common oil-water contact, in many cases, is attributed
to fractures joining the reservoirs. Also, the oil-water contact is often tilted as a result of hydrodynamic
flow (Stone, 1967).

5. Generalised Stratigraphy of Bighorn Basin (http://www.wsgs.wyo.gov/energy/oil-gas-basins)
Oil and gas bearing formations in Bighorn basin
In the Bighorn basin, oil has been produced primarily from the eolian and marine Tensleep Sandstone
(Middle Pennsylvanian to Lower Permian age), production also comes from the Madison Limestone
(Mississippian), Phosphoria Formation (Early Permian) and Amsden Formation (Lower Pennsylvanian).
Oil and Gas Fields in Bighorn Basin
There are over 145 demarcated oil and gas fields in Bighorn basin. The major ones are Oregon Basin,
Grass Creek, Elk Basin, Garland, Hamilton Dome, Cottonwood Creek, Little Buffalo Basin, Byron and
Frannie.

6. Oil and Gas fields in the Bighorn Basin, Wyoming
Table showing the oil/gas fields in the western margin of Bighorn Basin
Field Name
Active
Oil
Wells
Active
Gas
wells
Average
Depth
Producing Formation
% Oil
Produced
% Gas
Produced
Elk Basin 172 168 4,023 Frontier, Tensleep 9.27 21.99
Silver Tip 39 88 6,348 Tensleep 0.20 3.25
Elk Basin South 4 24 6,466 Tensleep 0.43 1.53
Bearcat 1 4 6,961 Phosphoria 0.08 0.28
Shoshone 12 12 3,832 Amsden 0.42 0.00
Cody 12 12 7,747 Tensleep 0.96 0.04
Oregon Basin
494 267 3,729 Tensleep, Embar,
Madison, Chugwater 23.13 23.45
Half Moon 22 16 3,639 Tensleep, Phosphoria 0.88 0.11
Hunt 20 0 4,270 Tensleep 0.13 0.00
Ferguson Ranch 18 0 4,096 Tensleep 0.45 0.00
Spring Creek South 86 86 4,368 Tensleep/Phosphoria 3.02 0.23
Pitchfork 47 0 4,309 Tensleep 4.26 0.74
Fourbear 31 0 3,816 Madison 0.88 0.00

7. Sheep Point 7 0 3,839 Tensleep 0.06 0.00
Sunshine North 36 0 3,989 Tensleep 0.57 0.00
Little Buffalo Basin 78 68 4,835 Tensleep, Embar 7.46 2.80
Gooseberry 24 0 6,258 Amsden 1.01 0.02
Grass Creek 220 210 2,579 Tensleep 7.32 0.97
Golden Eagle 1 0 6,946 Phosphoria 0.20 0.20
Hamilton Dome 183 0 3,195 Tensleep, Phosphoria 8.69 0.24
Little Sand Draw 3 0 6,350 Tensleep 0.80 0.06
Gebo 35 35 5,097 Tensleep 1.38 0.06
Murphy Dome 27 0 4,785 Tensleep 0.74 0.14
Kirby Creek 16 0 1,862 Tensleep 0.15 0.07
Black Mountain 36 0 3,529 Madison 1.19 0.15
Heart Mountain 2 9 4,783 Fuson 0.01 2.47
Lake Creek 11 0 3,686 Tensleep 0.27 0.01
Neiber Dome 3 0 9,875 Tensleep 0.06 0.06
Packsaddle 1 0 4,000 Tensleep 0.05 0.05
Oregon Basin West 3 1 4,247 Tensleep 0.04 0.03
Oregon Basin
Southeast
0 5 6,985
Muddy 0.00 1.48
31 1644 1005 74.12 60.42
Table showing the oil/gas fields in the eastern margin of Bighorn Basin
Field Name
Active
Oil
Wells
Active
Gas
wells
Average
Depth
Producing Formation
% Oil
Produced
% Gas
Produced
Frannie 36 0 3,389 Tensleep 3.31 0.14
Sage Creek 19 0 3,651 Tensleep 0.61 0.00
Homestead 3 0 4,529 Tensleep 0.17 0.00
Byron
71 61 4,803 Tensleep, Peay, Madison,
Amsden 4.27 0.28
Garland 245 237 3,717 Madison 8.40 4.70
Alkali Anticline 7 0 5,485 Madison 0.26 0.02
Spence Dome 47 0 680 Madison 0.10 0.00
Greybull 0 0 1,038 Peay 0.03 0.00
Lamb 4 1 2,784 Madison 0.07 0.06
Torchlight 17 18 1,796 Madison 0.57 0.09
Manderson 36 26 6,230 Phosphoria 0.22 3.69
Bonanza 5 0 2,561 Tensleep 0.23 0.00
Enigma 8 0 4,860 Amsden 0.40 0.06
Five Mile 15 18 10,305 Thermop 0.12 6.43

8. Hidden Dome 21 1 3,099 Tensleep, Madison 0.64 0.20
Cottonwood
Creek
97 121 8,141
Phosphoria, Tensleep 2.76 3.61
Frisby South 11 6 10,981 Phosphoria 0.74 0.49
Rattlesnake 16 9 10,887 Phosphoria 0.59 0.66
Frisby South 11 6 10,981 Phosphoria 0.74 0.49
Slick Creek 4 3 10,026 Phosphoria 0.30 0.60
No Water
Creek
11 7 10,814
Phosphoria 0.31 0.29
South Fork 1 0 10,527 Phosphoria 0.08 0.04
Marshall 3 1 10,137 Phosphoria 0.08 0.29
23 688 515 25.00 22.14
The above two tables show that the oil fields in the western part of Bighorn Basin produced higher oil
and gas compared to the fields in the eastern flanks of the basin. In the western flanks oil and gas
occur at an average depth of 4500 ft., whereas in the eastern margins of the basin the average depth
at which oil and gas occurs is 6000 ft and in the southeast corner the depth is nearly 10,000 ft.
Oil Production in fields
The Oregon Basin field is the most prolific oil producer in the Bighorn Basin and accounts for nearly
24% of all the crude oil produced in the basin from 1978 to August 2018.

9. Gas production in fields
The Oregon Basin field is the most prolific natural gas producer in the Bighorn Basin and accounts for
nearly 25% of all the gas produced in the basin from 1978 to August 2018.
Oregon Basin oil/gas field
The Oregon Basin field is about 9 to 10 miles long and 5 miles wide. The basin comprises of North
dome and south dome.
Distribution of oil wells in the north and south domes of Oregon Basin field and the heat map
showing oil production

10. The heat map further shows that the north dome of Oregon basin yielded more oil compared to the
southern part. There are nearly 494 active oil and 267 active gas wells as on August 2018. Whereas
the southern dome recorded higher gas output.
Distribution of oil wells in the north and south domes of Oregon Basin field and the heat map
showing gas production
E-W Cross section across the northern dome of Oregon Basin field

11. Producing Formations in Oregon Basin Field
Producing Formation
Total
Wells
% Oil
produced
% Gas
Produced
TENSLEEP 403 45.87 14.13
EMBAR 440 42.17 23.94
MADISON 110 10.56 5.96
DARWIN 40 0.99 0.30
PHOSPHORIA 8 0.20 0.00
EMBAR-TENSLEEP 13 0.07 0.03
DARWIN-MADISON 1 0.04 0.01
EMBAR-TENSLEEP-DARWIN 2 0.03 0.01
CHUGWATER 60 0.02 42.92
TENSLEEP-MADISON 1 0.02 0.00
FLAT HEAD 5 0.01 0.75
GROS VENTRE 3 0.01 1.93
MUDDY 6 0.01 3.17
TENSLEEP-DARWIN 1 0.00 0.01
FRONTIER 12 0.00 3.96
CLOVERLY 5 0.00 1.62
FRONTIER 1-2-3 1 0.00 0.01
SUNDANCE 1 0.00 0.19
DAKOTA 3 0.00 0.14
FRONTIER & MUDDY 1 0.00 0.12
LAKOTA 6 0.00 0.77

12. Nearly 98 % of oil in Oregon Basin was produced from Tensleep sandstone, Embar and Madison
Formations. Whereas, 81 % of gas was produced from Chugwater, Embar Formations and Tansleep
sandstone.
Oil and Gas production in Oregon Basin field

13. Oil production in Oregon Basin has decreased over the years and is approximately producing
2,000,000 BBls per year for the last 7 to 8 years and is likely to continue producing the same amount
of oil for the next few years. Gas production, on the other hand, has varied over the years with a peak
in 1998.
Source Rock
Two distinct source rock systems have been identified for the oil in Bighorn basin: (1) Permian
Phosphoria Formation and (2) Cretaceous Mowry, Frontier, Mesaverde and Meeteetse Formations
(https://certmapper.cr.usgs.gov/data/ noga95/prov34/text/prov34.pdf). The Phosphoria sourced oil is
characterized by low API gravity and low to moderate sulfur whereas oils from Cretaceous and Tertiary
formations have higher API gravities and low to absent sulfur content
(https://pubs.usgs.gov/fs/2008/3050/pdf/FS08-3050_508.pdf). The oil from Oregon Basin has an
average API of 20.78. Studies on sulfur content and API values have suggested that the Oregon Basin
oils are Type-II Kerogen and are sourced from Permian Phosphoria Formation
(https://www.researchgate.net/figure/Sulfur-content-versus-gravity-of-oils-from-Bighorn-Basin-this-
study-and-Orr-1974_fig10_260725180).
Migration of Oil
The Phosphoria-derived oils largely migrated into the Bighorn Basin from the west and have partly
cracked to gas with increasing thermal maturity. The migration of oil is thought be during Late
Cretaceous to Early Eocene Laramide orogeny (Blackstone, 1963).

14. Reservoir Rocks
The primary reservoir rocks in the Oregon basin are Tensleep sandstone, Embar, Madison and
Chugwater Formations.
Trap Rocks
Phosphoria Formation-sourced hydrocarbons accumulated in the Bighorn Basin in stratigraphic traps
created primarily by up dip facies change, pinch out and truncation of reservoir carbonates, and by
uneven Phosphoria Goose Egg truncation of the underlying Pennsylvanian Tensleep Sandston (Lawson,
M., Formolo. M. J., Summa, L. & Eiler, J. M. 2018).
Oil & Gas Producing Companies in Oregon Basin
Operator
% Oil
Produced
% Gas
Produced
Total
Wells
Wells in
2018
Coastal Chem Inc 0.00 0.35 2 0
Cumberland Operating Llc 0.01 5.52 5 5
Husky Oil Company 0.00 0.00 1 0
Marathon Oil Company 1.15 1.62 36 0
Merit Energy Company 98.65 92.39 538 413
Phoenix Production Company 0.19 0.13 6 6
The Merit Energy Company of Texas is the major operator and accounts for 98.65 % of Oil and 92.39%
of Gas production in the Oregon Basin field.
Acknowledgement
All location, geological and production data is taken from Wyoming Oil & Gas Conservation
Commission website (http://pipeline.wyo.gov/).
References
Blackstone, D.L., Jr., 1963, Development of geologic structures in central Rocky Mountains, in Childs,
O.E., and Beebe, B.W., eds., Backbone of the Americas, Tectonic history from pole to pole: Tulsa, Ok.,
American Association of Petroleum Geologists Memoir, p. 160-179.
Fox, J.E., and Dolton, G.L., 1996, Petroleum geology of the Bighorn Basin, north-central Wyoming and
south-central Montana, in Bowen, C.E., Kirkwood, S.C., and Miller, T.S., eds., Resources of the Bighorn
Basin: Casper, Wy., Wyoming Geological Association, 47th annual field conference, Guidebook, p. 19-
39.

15. Lawson, D.E., and Smith, J.R., 1966, Pennsylvanian and Permian influence on Tensleep oil
accumulation, Bighorn Basin, Wyoming: American Association of Petroleum Geologists Bulletin, v. 50,
no. 10, p. 2197-2220.
Lawson, M., Formolo. M. J., Summa, L. & Eiler, J. M., 2018, Geochemical applications in petroleum
systems analysis: New constraints and the power of integration Geological Society London Special
Publications 468(1): SP 468.6
Stone, D.S, 1967, Theory of Palaeozoic oil and gas accumulation in Bighorn Basin, Wyoming: American
Association of Petroleum Geologists Bulletin, v. 51, no. 10, p. 2056-2114.