--> ABSTRACT: Review of Geology and Petroleum Potential of North-Central Montana, by T. S. Dyman and E. K. Maughan; #91033 (2010)

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Review of Geology and Petroleum Potential of North-Central Montana

T. S. Dyman, E. K. Maughan

An assessment of undiscovered petroleum resources of the north-central Montana petroleum province was conducted using a play analysis approach for the Federal lands Assessment Program. More than 340 million bbl of oil and 1,175 bcf of gas have been produced from the 33 major fields in the province since 1919 when Cat Creek field was discovered in Petroleum County.

Late Precambrian through Tertiary strata can be grouped into four major exploration plays: upper Proterozoic-lower Paleozoic oil play, Devonian-Mississippian oil and gas play, Jurassic-Lower Cretaceous oil and gas play, and Upper Cretaceous gas play. A fifth play, the Pennsylvanian oil play, is not significant because Pennsylvanian rocks are absent, except in the extreme southern part of the region where only minor amounts of oil have been produced in this province.

Organic, carbon-rich hydrocarbon source rocks are thermally immature to mature. Most Upper Cretaceous gas is biogenic methane from thermally immature source rocks.

Many Laramide structures enhance petroleum traps within the province. Even where structural closure exists, as on the crest of the Sweet-grass arch, most reservoirs are partly controlled by facies changes in reservoir rocks. Paleozoic carbonate reservoirs include evaporite-carbonate cycles, bioclastic carbonate banks, and paleokarst systems that have been enhanced by dolomitization. Clastic reservoirs include marine and nonmarine sandstones with lateral and vertical facies variations.

Since 1915, more than 15,000 wells have been drilled in the province; total footage drilled is more than 26 million ft, of which approximately 11 million ft are from exploratory wells. The total ultimate recoverable oil assessment approximates 370 million bbl, of which 13 million bbl are in fields smaller than 1 million BOE. The total ultimate recoverable gas approximates 2,211 bcf, of which 400 bcf are in fields smaller than 1 million BOE. More than 2,000 bcf of the total ultimate recoverable gas occurs in the Jurassic-Lower Cretaceous and Upper Cretaceous plays.

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