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Salt deposits in the Powder River basin of Wyoming occur in the Late Permian Ervay Member of the Goose Egg Formation which was deposited in a redbed-evaporite trend extending from the Williston basin of North Dakota to the Alliance basin of Nebraska and Wyoming. However, only remnants of the once extensive Ervay salt remain in the Powder River basin, with major salt dissolution events occurring during Late Jurassic and Early Cretaceous. Subsidence and deposition at the surface were contemporaneous with subsurface salt dissolution except in areas where uplift and erosion were occurring. The presence or absence of Ervay salt and the relationship to overlying syndepositional strata can be seen readily and mapped using borehole logs or seismic data.
Earliest dissolution of the Ervay salt occurred in the Jurassic, during regional uplift and erosion of the overlying Triassic Chugwater Formation in the present Hartville uplift and southeastern Powder River basin areas. Thickness variations of the Canyon Springs and Stockade Beaver members of the early Late Jurassic Sundance Formation, which unconformably overlie the deeply eroded Chugwater Formation, may be related in part to dissolution of the Ervay salt. Extensive salt dissolution, synsubsidence, and syndeposition occurred throughout most of the Powder River basin during latest Jurassic and Early Cretaceous. Evidence of this is seen in thick trends of the Morrison, Lakota, Dakota, or Muddy formations overlying areas of Ervay salt collapse. One area escaping extensive dissolution i the Early Cretaceous was the eastern Belle Fourche arch, which trends northeast across the middle of the Power River basin. Here the Lakota, Dakota, and Muddy formations are thin over areas with underlying Ervay salt, but thicken rapidly in areas of salt collapse.
Many producing fields from the Mowry, Muddy, and Dakota formations exhibit either rapid stratigraphic changes syndepositional to salt collapse or fracture-enhanced reservoir quality due to postdepositional salt collapse. Major Muddy accumulations occurring in areas of local Ervay salt collapse include Kitty, Hilight, Fiddler Creek, and Clareton which have produced jointly over 172 million bbl of oil. The relationship of Ervay salt dissolution to Lower Cretaceous deposition can be exploited as an effective exploration tool.
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