Mowry Shale, Powder River Basin, Wyoming
By Zachary Hollon, Workhorse Geologic LLC, USA
The Lower-Upper Cretaceous, Albian-Cenomanian-aged Mowry Shale in the Powder River Basin of northeastern Wyoming and southeastern Montana (Figures 1 and 2) is a siliceous, organic-rich, dark-gray to black marine mudstone interbedded with bentonite, sandstone, and silt.[1][2] [3]. Radiolaria tests, Fish scales, fish teeth, fish bones, fecal pellets, inoceramus debris, and ammonites are found in the bedding planes and silt-laminae of the Mowry Shale throughout Wyoming[4] (Davis, 1970). Calcareous cone-in-cone concretions are seen in core and outcrop[5]. Numerous bentonite beds up to 3 ft thick in the Mowry Shale allow for the precise recognition of time equivalent strata over vast areas[3]. In outcrop, the Mowry Shale has a distinct profile and weathers to a dark to light gray, is hard, and often has large jointing sets. The Mowry Shale is ranges in thickness from 150 to 250 ft and is a significant source rock for the Cretaceous reservoirs in the Powder River Basin.
Exploration and production from the Mowry Shale has been periodic and slow. As the writing of this, there are 47 completed horizontal wells in the Mowry, 17 of which have been drilled since 2018, and most of the wells are south of the Belle Fourche Arch in Campbell and Converse Counties. The 2006-2015 one-mile laterals rarely made commercial wells, plagued by drilling and completion issues from the thick bentonites that are found throughout the Mowry. EOG’s return to the Mowry Shale in 2018 with modern slickwater completions designs and two-mile laterals proved the Mowry to be a commercial oil play in their core acreage, renewing interest from basin operators. In 2019, the Mowry had its largest production, producing ~986,000 BO, ~9.4 BCF, and ~1,800,000 BW. In 2020, the Mowry production decreased slightly, producing ~960,000 BO, ~8.5 BCF, and ~1,200,000 BW[6].
The Mowry’s depth and mixed Type II/III kerogen makes it more gas and condensate prone than the shallower, Type II kerogen Niobrara. The prevalence of several shallower, oilier reservoirs above the Mowry reduces the likelihood of full-scale development any time soon. Operators recognize how prolific the Mowry Shale can be but will develop other reservoirs first while delineating the Mowry Shale with one-off wells until it can be proven over a larger area.
As of the July of 2021, there are 11 rigs drilling in the Powder River Basin, and it is expected for the Mowry Shale’s production to slowly increase as EOG and other operators delineate this reservoir.
References
- ↑ Burtner, R., and M. Warner, 1984, Hydrocarbon generation in Lower Cretaceous Mowry and Skull Creek shales of the northern Rocky Mountain area: Hydrocarbon Source Rocks of the Greater Rocky Mountain Region, Rocky Mountain Association Geologists.
- ↑ Davis, H. R., C. Byers, and L. Pratt, 1989, Depositional mechanisms and organic matter in Mowry Shale (Cretaceous), Wyoming: AAPG Bulletin, vol. 9, p. 1103–1116.
- ↑ 3.0 3.1 Nixon, R. P., 1973, Oil Source Beds in Cretaceous Mowry Shale of Northwestern Interior United States: AAPG Bulletin, vol. 57, no. 1, p. 136–161
- ↑ Anderson, A., and B. Kowallis, 2005, Storm deposited fish debris in the Cretaceous Mowry Shale near Vernal, Utah: Utah Geological Association Publication 33, p. 125–130.
- ↑ Hollon, Z.G., 2014, Elemental chemostratigraphy and reservoir properties of the Mowry Shale in the Bighorn and Powder River basins, Wyoming, USA: MS thesis, Colorado School of Mines, 156 p.
- ↑ Wyoming Oil and Gas Conservation Commission
Other Sources
- Anna, L., 2009, Geologic assessment of undiscovered oil and gas in the Powder River Basin Province, Wyoming and Montana: U.S. Geological Survey Digital Data Series DDS–69–U, p. 93.
- Craddock, W. H., R. M. Drake II, J. C. Mars, M. D. Merrill, P. D. Warwick, M. S. Blondes, M. A. Gosai, et. al, 2012, Geologic framework for the national assessment of carbon dioxide storage resources—Powder River Basin, Wyoming, Montana, South Dakota, and Nebraska, in P. D. Warwick and M. D. Corum, eds., Geologic framework for the national assessment of carbon dioxide storage resources: U.S. Geological Survey Open-File Report 2012–1024–B, 30 p.