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Outcrop, petrographic, and subsurface analysis of the Sappington Member, Three Forks Formation, and Cottonwood Canyon Member, Lodgepole Formation, central Montana
Lockwood, Thomas D.
Lockwood, Thomas D.
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2016
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2016-11-17
Abstract
The Sappington Member of the Three Forks Formation (latest Devonian) and the overlying Cottonwood Canyon Member of the Lodgepole Formation (Early Mississippian) outcrop in various locations of the Disturbed Belt throughout south-central Montana. These two members are correlative, lithologically similar, and deposited contemporaneously with the Bakken Formation in the Williston Basin (North Dakota). Three outcrops of the Sappington and Cottonwood Canyon members near Bozeman, Montana were examined in August, 2015 to construct stratigraphic logs and collect samples for petrographic and x-ray diffraction analysis. Interpretation of data yielded identification of facies, depositional environments, and post depositional structural and diagenetic alterations. The Sappington and Cottonwood Canyon members were also identified on 64 publically available well logs east of Bozeman in the Crazy Mountain Basin subsurface. Since the Bakken Formation is known for having prolific unconventional oil and gas production in the Williston Basin, an integrated outcrop, petrographic, and subsurface analysis of the Sappington and Cottonwood Canyon members was performed to assess if they might form a viable petroleum system in the Crazy Mountain Basin. The Sappington Member was deposited between 364.5 Ma and 361.5 Ma, and the Cottonwood Canyon Member unconformably overlies the Sappington Member and was deposited at ~355 Ma. The Sappington Member consists of a basal lower shale unit that is correlative to the Bakken Formation lower shale member. The upper part of the Sappington Member includes a nodular limestone unit, lower ledge unit, middle recessive unit, and upper ledge unit, in ascending order. These are correlative to the Bakken Formation middle member. The unconformably overlying Cottonwood Canyon Member is correlative with the Bakken Formation upper shale member. While the Bakken Formation was deposited within a intracratonic basin, the Sappington and Cottonwood Canyon members were deposited in a dynamic back-bulge basin of the Laurentian continental shelf. This back-bulge basin was a product of the eastward compression of the Antler Orogeny and resulting continental shelf buckling during the latest Devonian to Early Mississippian. Tectonic inversion of Precambrian-age fault blocks normal to the continental shelf, as a result of Antler orogenic compression, caused rapid development of shelf paleotopography and subsequent high erosion rates in a postulated subtropical equatorial paleoclimate. Dynamic tectonics, paleoclimate, and eustatic sea level all had significant influences on facies development within the Antler backbulge basin during this time. Facies identified within the Sappington and Cottonwood Canyon members include a (1) silty carbonaceous shale, (2) clay- and quartz-rich recrystallized calcite, (3) carbonaceous fossiliferous shale, (4) oncolitic brachiopod floatstone, (5) calcareous silty dolomite, (6) burrowed calcareous siltstone, and (7) dedolomitized calcareous sandstone. Interpreted depositional environments within the backbulge basin range from anoxic deep subtidal to shallow subtidal carbonate banks, shallow restricted brackish basin waters, and nearshore, shallowest subtidal tidal flats. Terrigenous erosion proximal to the basin margin and resultant freshwater runoff into the back bulge basin played a significant role in carbonate shelf clastic dilution and mixed-water dolomitization of micrite mud. Subsequent burial of the Sappington Member resulted in significant dedolomitization of certain upper Sappington Member facies. Subsurface mapping of these two members in the Crazy Mountain Basin yielded structure and isochore maps helpful in ascertaining areas of potential Sappington and Cottonwood Canyon member hydrocarbon play development. The Sappington Member is truncated to the east of Bozeman by the Cottonwood Canyon Member, and is restricted to a ~50 mile wide, southwest-northeast trending western corridor of the Crazy Mountain Basin. The Sappington Member spans ~9,000 square miles in the subsurface and ranges from 21-99 feet thick. The Cottonwood Canyon Member spans ~20,000 square miles in the subsurface, and ranges from 2 to 78 feet thick. Source rock analysis of a Crazy Mountain Basin core sample indicates that the Sappington Member lower shale unit and Cottonwood Canyon Member may be mature at depths greater than 7,000 feet. Shales may also be mature to overmature at shallower depths near areas of igneous intrusion. In terms of the upper Sappington Member, areas of the best unconventional reservoir will be where calcite precipitation (dedolomitization) has been minor within the dedolomitized calcareous sandstone facies. Areas of best unconventional play development are likely where thick upper Sappington Member reservoir facies are present between thick sections of the Sappington Member lower shale unit and Cottonwood Canyon Member source rocks. Mapping results indicate that this occurs at depths greater than 7,000 feet in northwest Sweetgrass County. The best areas of upper Sappington Member conventional reservoirs may be located at depths greater than 7,000 feet within thrusted, asymmetric anticlines along the western margin of the Crazy Mountain Basin formed by Laramide age compression. However, conventional reservoir quality depends on the relative timing of Sappington Member dedolomitization and Laramide compression, which remains unknown.
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