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AAPG Bulletin

Abstract

(Begin page 131)

AAPG Bulletin, V. 85, No. 1 (January 2001), P. 131-148.

Copyright ©2001. The American Association of Petroleum Geologists. All rights reserved.

Unconformity-related chert/dolomite production in the Pennsylvanian Amsden Formation, Wolf Springs fields, Bull Mountains basin of central Montana

Glen A. Luebking,1 Mark W. Longman,2 W. Joseph Carlisle3

1Savannah Exploration, Inc., 10649 W. Glasgow Ave, Littleton, Colorado, 80127; email: [email protected]
2Consulting Geologist, 701 Harlan #E-69, Lakewood, Colorado, 80214; email: [email protected]
3Consulting Geologist, 2416 Avalon Road, Billings, Montana, 59102; email: [email protected]

AUTHORS

Glen A. Luebking is an independent/consulting geologist residing in Littleton, Colorado. He received a B.S. degree from Southern Illinois University-Carbondale in 1980. He worked as an exploration geologist for Meridian Oil and HS Resources, Inc. prior to founding Savannah Exploration, Inc. in 1996. His focus is generating oil and gas prospects in the Rocky Mountain region, Illinois basin, and Michigan basin.

Mark W. Longman is a consulting geologist who specializes in the study of carbonate rocks and reservoirs. He has worked on a wide variety of projects ranging from specific core studies on individual wells and hydrocarbon reservoirs to regional studies covering entire basins, states, and countries. His specialties include interpretation of carbonate depositional facies and diagenesis and tying these interpretations to wire-line logs. He received a B.A. degree from Albion College (Michigan) in 1972 and a Ph.D. from the University of Texas at Austin in 1976. Before becoming a consultant in 1984, he was a research geologist with Cities Service Company's research laboratory in Tulsa, and he later worked with Coastal Oil and Gas Corporation and Buttercup Energy in Denver.

W. Joseph (Joe) Carlisle is a geological consultant in Billings, Montana. Joe received his B.S. degree in geology from Weber State in 1974 and received an M.S. degree from the University of Wyoming in 1979. Joe has worked as an exploration/development geologist for Conoco Inc., Meridian Oil, American Hunter Exploration, and Equitable Resources Inc. during his career. His interests include regional sequence stratigraphy, carbonate and clastic sedimentology, subsurface pressure compartments and seals, reservoir petrography, and the application of each to exploration for hydrocarbons. He is a member of AAPG and the Montana Geological Society.

ACKNOWLEDGMENTS

Glen A. Luebking extends special thanks and appreciation to John R. Fanshawe (deceased) who provided lively discussion and debate concerning the Amsden Formation. Also, thanks to Thomas J. Birmingham, HS Resources, Inc., and Raymond Sorenson, Anadarko Petroleum, for critical reviews of this article. Thanks, too, to John R. Mitchell, Rutherford Exploration, Inc., for the many hours spent discussing and debating exploration philosophies and ideas in the search for new oil fields.

ABSTRACT

Wolf Springs field (north and south pools) and South Wolf Springs field (a.k.a. Wolf Springs fields), located in Yellowstone County, Montana, were discovered in 1955 and 1957, respectively, and have produced more than 5.7 million bbl oil from the Pennsylvanian Amsden Formation. Amsden reservoir rocks in the area are fractured and brecciated cherts and dolomites that occur in several laterally persistent and mappable zones.

The Amsden was deposited in a peritidal to sabkha setting where evaporite minerals, mainly anhydrite, were once common. These evaporites were partly replaced by silica (chalcedony and chert) soon after deposition. Later dissolution of the remaining evaporites soon after the silicification event, or during the pre-Middle Jurassic unconformity (PMJU), produced the solution-collapse chert breccias that now serve as the best reservoir facies in the field. Subtle variations in the diagenetic history of these breccias was a major factor in shaping reservoir quality.

The Wolf Springs fields are unconformity-related combination structural and stratigraphic traps. The fields are located on a structural closure on the Custer anticline, where porosity and permeability development exhibit a northeast-southwest orientation perpendicular to structural strike of the anticline. The solution-collapse breccias pinch out laterally into either dense dolomites or anhydrite-plugged collapse breccias. The overlying shaly dolomite breccia of the Botts member (informal name) located just below the Piper unconformity and the Jurassic Piper Limestone provide an effective top seal.

(Begin page 132)

Understanding the geographic distribution of the chert/dolomite zones provides a key to exploration for these reservoirs. This must be coupled with analysis of available rocks and drillstem-test data and the integration of the regional hydrodynamic forces affecting the Amsden. These exploration tools should lead to the discovery of new Amsden reserves in the Bull Mountains basin and Central Montana platform.

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