Geologic Background
The Three Forks Formation is a mixed carbonate-siliciclastic unit comprised of claystone, dolomitic to siliciclastic mudstone, dolostone, conglomerates/breccias with lesser amounts of anhydrite and sandstone (Bottjer et al., 2011; Droege, 2014; Franklin and Sarg, 2018; Garcia-Fresca et al., 2018). The Three Forks lacks fossil assemblages, making direct biostratigraphic age constraints impossible. However, biostratigraphy from the overlying Bakken and underlying Birdbear Formations dictate that the Three Forks was deposited during the Late Devonian (Famennian) (Fig. 1) (Sandberg and Hammond, 1958; Holland et al., 1987; Thrasher, 1987). The Williston Basin was connected to the open ocean during the Devonian through a linear trough-like depression referred to as the Elk Point basin. The Sweetgrass arch formed a paleo-topographic high within the Elk Point Basin in the Late Devonian, restricting communication between the open ocean and the Williston Basin during Three Forks deposition (Fig. 2) (Franklin and Sarg, 2018; Garcia-Fresca et al., 2018).
Figure 1. Wireline log example of the Three Forks Formation with various informal stratigraphic nomenclature systems.
Figure 2. Paleogeographic map during the late Devonian.
Multiple depositional settings have been proposed for the Three Forks Formation. Most recently, Franklin and Sarg (2018) interpreted that the Three Forks was deposited within an intrashelf siliciclastic-enriched basin with a variable, restricted marine connection. Alternatively, Garcia-Fresca et al. (2018) proposed that Three Forks deposition occurred within a continental setting with minimal to negligible marine influence, comparable to a playa lake or continental sabkha setting.
Multiple stratigraphic nomenclature systems have been developed and utilized for the Three Forks Formation within the Williston Basin (Fig. 1). Christopher (1961, 1963) developed a six-unit system reflecting broadly alternating dolomite-anhydrite versus more argillaceous horizons in the Three Forks equivalent Torquay Formation across southern Saskatchewan, which was later correlated into western North Dakota (LeFever and Nordeng, 2008; LeFever et al., 2009; LeFever et al., 2011). Bottjer et al. (2011) adopted a more simplified three-member stratigraphic nomenclature system (upper, middle, and lower members) that subdivides the section utilizing the tops of the high gamma-ray equivalent intervals of Christopher units 3 and 5 (Fig. 1). Additionally, a five-member system has also been utilized in some publications (LeFever et al., 2013; 2014). As well as a four-subunit system referred to as Three Forks 1 to Three Forks 4 in descending order (Hogancamp et al. (2019). Additionally, some companies operating in the Bakken-Three Forks have adopted a 4-bench terminology system which approximately corresponds with four generalized stratigraphic/reservoir target intervals (Fig. 1). A more complete summary and review of correlation comparisons of most of the above stratigraphic systems can be found in Nesheim (2019a).
Oil & Gas Resources
The Three Forks Formation is a prolific unconventional oil and gas reservoir within western North Dakota and has been the target of approximately 8,000 horizontal wells through ~mid-2023 (Fig. 3). The Three Forks Formation is part of the Bakken Petroleum System, in which hydrocarbons have been generated by the Lower Bakken shale and expelled downwards into the underlying Three Forks reservoir(s) (Nesheim, 2019b). Exploration and development in the Three Forks primarily began in the late 2000s, following the emergence of the Bakken Formation in the Williston Basin. Most Three Forks horizontal wells have been drilled and completed in the upper Three Forks, which is closest stratigraphically to the lower Bakken shale. Meanwhile, approximately 340 wells have been drilled and completed in the middle Three Forks and less than 50 wells in the lower Three Forks (Fig. 1) (Nesheim, 2023).
Figure 3. Subsurface extent map of the Bakken and Three Forks Formations within North Dakota overlain by the distribution of Three Forks productive wells (black-green-yellow symbols). Black circles indicate wells indicate the distribution of upper Three Forks horizontal wells (1st bench, likely includes some wells within the Pronghorn Member of the Bakken Formation towards southwestern North Dakota). The green squares show the partial distribution of middle Three Forks horizontal wells (2nd bench) and the yellow diamonds show the distribution of lower Three Forks horizontal wells (3rd and 4th benches). Note that many of the wells clustered near southwestern North Dakota have likely targeted the Pronghorn Member of the Bakken Formation but were permitted as Three Forks wells.
Bottjer, R.J., Sterling, R., Grau, A., Dea, P., 2011. Stratigraphic relationships and reservoir quality at the Three Forks-Bakken unconformity, Williston Basin, North Dakota. In: Robinson, L., LeFever, J., Gaswirth, S. (Eds.), Bakken-Three Forks Petroleum System in the Williston Basin: Rocky Mountain Association of Geologists Guidebook, pp. 173–228.
Christopher, J. E., 1961, Transitional Devonian-Mississippian Formation of southern Saskatchewan. Regina, Saskatchewan, Canada, Saskatchewan Mineral Resources Geological Report 66, 103 pp.
Christopher, J.E., 1963, Lithological and geochemical aspects of the Upper Devonian Torquay Formation, Saskatchewan: Journal of Sedimentary Petrology, v. 33, p. 5-13.
Droege, L.A., 2014. Sedimentology, Facies Architecture, and Diagenesis of the Middle Three Forks Formation – North Dakota, U.S.A. MS thesis. Colorado State University, Fort Collins, Colorado, pp. 150.
Franklin, A., and Sarg, J.F., 2018, Storm-influenced intrashelf systems: sedimentological characterization of the Famennian Three Forks Formation, Williston Basin, U.S.A.: Journal of Sedimentary Research, v. 88, p. 583-612. DOI:10.2110/jsr.2018.24
Garcia-Fresca, B., Pinkston, D., Loucks, R.G., and LeFever, R., 2018, The Three Forks playa lake depositional model: implications for characterization and development of an unconventional carbonate play: AAPG Bulletin, v. 102, no. 8, p. 1455-1488. DOI:10.1306/12081716510
Gaswirth, S.B., Marra, K.R., Cook, T.A., Charpentier, R.R., Gautier, D.L., Higley, D.K., Klett, T.R., Lewan, M.D., Lillis, P.G., Schenk, C.J., Tennyson, M.E., Whidden, K.J., 2013. Assessment of undiscovered oil resources in the Bakken and Three Forks formations, Williston Basin Province, Montana, North Dakota, and South Dakota, 2013: U.S. Geol. Surv. Fact Sheet 2013–3013, 4.
Gaswirth, S.B., Marra, K.R., 2015. U.S. Geological Survey 2013 assessment of undiscovered resources in the Bakken and Three Forks formations of the U.S. Williston basin Province. AAPG (Am. Assoc. Pet. Geol.) Bull. 99 (4), 639–660. DOI: 10.1306/08131414051
Hogancamp, N.J., Hohman, J.C., and Guthrie, J.M., 2019, Stratigraphy and lithology of the Logan Gulch Member of the Three Forks Formation in the type area and its regional stratigraphic framework: Stratigraphy, vol. 16, no. 3, p. 145-164.
Holland, F.D., Jr., Hayes, M.D., Thrasher, L.C., and Huber, T.P., 1987, Summary of the biostratigraphy of the Bakken Formation (Devonian and Mississippian) in the Williston Basin, North Dakota: Fifth International Williston Basin Symposium, Saskatchewan Geological Society, Special Publication 9, p. 68–76.
LeFever, J.A., and Nordeng, S.H., 2008, Correlation cross-section - Three Forks Formation: North Dakota Geological Survey Geological Investigations no. 65, 1 plate. GI-65
LeFever, J.A., LeFever, R.D., and Nordeng, S.H., 2011, Revised nomenclature for the Bakken Formation (Mississippian-Devonian), North Dakota, in J. W. Robinson, J.A. LeFever, and S.B. Gaswirth eds., The Bakken-Three Forks petroleum system in the Williston Basin: Rocky Mountain Association of Geologists Guidebook, p. 11-26.
LeFever, J.A., LeFever, R.D., and Nordeng, S.H., 2013, Role of nomenclature in pay zone definitions – Three Forks Formation, North Dakota: North Dakota Geological Survey Geologic Investigations no. 165. GI-165
LeFever, J.A., LeFever, R.D., and Nordeng, S.H., 2014, Reservoirs of the Bakken Petroleum System: A core-based perspective: North Dakota Geological Survey Geologic Investigations no. 171. Part 1 | Part 2 | Part 3
LeFever, R.D., LeFever, J.A., and Nordeng, S.H., 2009, Correlation cross-sections for the Three Forks Formation, North Dakota: North Dakota Geological Survey Report of Investigations no. 108, 9 sheets.
Sheet 1 | Sheet 2 | Sheet 3 | Sheet 4 | Sheet 5 | Sheet 6 | Sheet 7 | Sheet 8 | Sheet 9
Marra, K.R., Mercier, T.J., Gelman, S.E., Schenk, C.J., Woodall, C.A., Cicero, A.D., Drake II, R.M., Ellis, G.S., Finn, T.M., Gardner, M.H., Hearon, J.S., Johnson, B.G., Lagesse, J.H., Le, P.A., Leathers-Miller, H.M., Timm, K.K., and Young, S.S., 2021, Assessment of Undiscovered Continuous Oil Resources in the Bakken and Three Forks Formations of the Williston Basin Province, North Dakota and Montana: U.S. Geol. Surv. Fact Sheet 2021-3058, 4 p.
Nesheim, T.O., 2019, Examination of downward hydrocarbon charge within the Bakken-Three Forks petroleum system – Williston Basin, North America: Marine and Petroleum Geology, Vol. 104, p. 346-360. DOI: 10.1016/j.marpetgeo.2019.03.016
Sandberg, C.A., and Hammond, C.R., 1958, Devonian system in Williston Basin and central Montana: American Association of Petroleum Geologists Bulletin, v. 42, no. 10, p. 2293-2334.
Thrasher, L., 1987, Macrofossils and stratigraphic subdivisions of the Bakken Formation (Devonian–Mississippian), Williston Basin, North Dakota, in Fischer, D.W., ed., Fifth International Williston Basin Symposium: Saskatchewan Geological Society, p. 53-67.
Last Updated: 01/12/2024.