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Petroleum Potential of Idaho-Wyoming Overthrust Belt

Lawrence E. Monley
Lawrence E. Monley
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January 01, 1971


The Idaho-Wyoming Overthrust belt is a part of the Rocky Mountain orogenic complex. The belt is an area of severely deformed Paleozoic and Mesozoic strata characterized by westward-dipping thrust faults. Eastward movement on these faults has resulted in considerable shortening of the sedimentary blanket by the placement of older rocks over younger rocks. Four or five major thrust faults, lesser thrusts, and folds, tear faults, and "post-thrusting" normal faults have resulted in the complex structure of the orogenic belt. Many untested surface anticlines are present within the area, and some of them may reflect closure in lower thrust plates.

The present width of the Overthrust belt is roughly coincident with the average eastern hinge line of a Paleozoic and early Mesozoic miogeosyncline whose depocenter was in south-central and southeastern Idaho. The transition from a thick marine section to a thinner shelf section in western Wyoming provides both source and reservoir beds.

Because of shows and evidence of porosity in test wells and/or surface exposures, the Mississippian Brazer-Madison, Pennsylvanian Weber-Wells, Triassic Thaynes and Nugget, and Jurassic Twin Creek strata are considered to be primary potential reservoirs.

Approximately 78 significant test wells have been drilled within the Overthrust belt, but only 16 of 66 surface anticlinal trends have been tested. Because asymmetry and greater complexity with depth have been the rule, most of these tests probably have not been on crestal positions at depth. Fewer still have been based on seismic data.

Although not now productive, the Idaho-Wyoming Overthrust belt meets the classic requirements of a petroleum province, i.e., the presence of reservoir source rock and caprock under structural deformation. A volumetric comparison of this area with a maturely explored area of Wyoming indicates that a "speculative" 3.3 billion bbl of recoverable reserves might be present in the area. Despite rugged topography and complex structure, an integrated exploration program could develop substantial reserves within the Overthrust belt.

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AAPG Memoir

Future Petroleum Provinces of the United States—Their Geology and Potential, Volumes 1 & 2

Ira H. Cram
Ira H. Cram
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American Association of Petroleum Geologists
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Publication date:
January 01, 1971




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