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'''By Zachary Hollon, Workhorse Geologic LLC, USA'''

'''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. (Nixon, 1973; Burtner and Warner, 1984; Davis et al., 1989). 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<ref>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.</ref> (Davis, 1970). Calcareous cone-in-cone concretions are seen in core and outcrop (Hollon, 2014). Numerous bentonite beds up to 3 ft thick in the Mowry Shale allow for the precise recognition of time equivalent strata over vast areas (Nixon, 1973). 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.

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. (Nixon, 1973; Burtner and Warner, 1984; Davis et al., 1989). 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 (Davis, 1970; Anderson and Kowallis, 2005). Calcareous cone-in-cone concretions are seen in core and outcrop (Hollon, 2014). Numerous bentonite beds up to 3 ft thick in the Mowry Shale allow for the precise recognition of time equivalent strata over vast areas (Nixon, 1973). 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 (Figure 3) (WOGCC, 2021).

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 (Figure 3) (WOGCC, 2021).

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.  

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.

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.

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.

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.