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The AAPG/Datapages Combined Publications Database

Wyoming Geological Association

Abstract


The Permian and Pennsylvanian Geology of Wyoming; 35th Previous HitAnnualNext Hit Field Conference Guidebook, 1984
Pages 281-294

Dissolution of Permian Salt and Mesozoic Syndepositional Trends, Central Powder River Basin, Wyoming

Donald L. Rasmussen, Daniel W. Bean

Abstract

Salt deposits in the Powder River Basin of Wyoming occur in the Late Permian Ervay Member of the Goose Egg Formation which was deposited as a part of a red bed-evaporite sequence. These beds extend from the Williston Basin of North Dakota and Montana across the Powder River Basin to the Alliance Basin of Wyoming and Nebraska. Major salt dissolution events occurred in Late Jurassic and Early Cretaceous, leaving only remnants of the once-extensive Ervay salt. Subsidence of the overlying strata and compensatory depositon at the surface were contemporaneous with subsurface salt dissolution. The presence or absence of Ervay salt and the relationship to overlying syndepositional strata can be mapped using borehole logs and/or seismic data.

The earliest dissolution of the Ervay salt in the Powder River Basin occurred in Medial Jurassic time near the Wyoming-Montana border, and in Late Jurassic time far to the southeast, along the Hartville Uplift. The earliest salt dissolution events are postulated to be initiated by local fracturing related to basement tectonics. This fracturing provided conduits for groundwater circulation which slowly removed the Ervay salt by dissolution. Dissolution ceased when the salt in the Ervay was locally completely removed or when groundwater circulation ceased in response to sealing of the system by the deposition of transgressive marine Mowry and Belle Fourche shales. One area escaping extensive dissolution in the Late Jurassic and Early Cretaceous was on the eastern Belle Fourche Arch, which extends in a northeast trend across the middle of the Powder River Basin. Here the Morrison, Lakota, and Dakota Formations are thin over areas with underlying Ervay salt, but thicken rapidly in areas of salt collapse. In a 12 township study area along the southern margin of the Belle Fourche Arch, borehole logs from more than 500 wells which penetrate Permian strata document rapid changes in the Ervay salt and overlying Mesozoic strata. In the Powder River Basin, many fields that produce from the Dakota, Muddy, and Mowry Formations exhibit either rapid stratigraphic changes caused by syndepositional salt dissolution or fracture-enhanced reservoir quality due to post-depositional salt collapse. Major Muddy accumulations occurring in areas of local Ervay salt collapse include Kitty, Hilight, Fiddler Creek, and Clareton fields which have jointly produced over 172 million barrels of Previous HitoilTop. The relationship between Ervay salt dissolution-collapse events and Early Cretaceous depositional trends can be exploited as an effective exploration tool.


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