--> ABSTRACT: Hydrocarbon Potential of Lower Duperow Formation as Cored in Hill and Chouteau Counties, Central Montana, by George C. Robertson and S. Robert Bereskin; #91033 (2010)

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Hydrocarbon Potential of Lower Duperow Formation as Cored in Hill and Chouteau Counties, Central Montana

George C. Robertson, S. Robert Bereskin

A paleotopographic ridge existed in north-central Montana during southward transgression of Souris River (Giventian) and lower Duperow (Frasnian) seas. This ridge represented a southeasterly extension of the Canadian Alberta Ridge, which served as a locus for reef development in western Alberta during the Devonian. Located on the back-reef shelf between the Cordilleran barrier reef and the intracratonic Williston basin, this ridge formed a regional high favorable to patch-reef development, much like the Alberta Ridge did for more extensive reef complexes in western Canada.

Extensive coring of lower Duperow and Souris River carbonates in four wells detailed several depositional facies and demonstrated widespread distribution of stromatoporoidal buildups up to 20 ft in individual thickness. Faunal assemblages indicate mostly normal marine conditions and potential for patch-reef development. Equally attractive reservoir facies comprised of cross-bedded detrital dolomite bars and/or channels, leached soil horizons, and bedded anhydrite-laminated carbonate sabkha facies were encountered in cored intervals. These reservoir facies reflect lateral stratigraphic variations in otherwise regionally extensive cyclic sequences. Each regional cycle is terminated by an anhydritic phase reflecting separate pulses in the overall transgressive advance of the Devonian sea . The capping anhydrite and lateral facies variation within each cycle provide numerous opportunities for stratigraphic traps.

Residual oil extracted from cored samples correlates well with Williston basin Red River-type oil. Source rock evaluation of Souris River carbonates coupled with diagenetic evidence of early oil migration into lower Duperow and Souris River reservoir facies indicate a favorable thermal history for oil preservation.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988