Dissolution of Permian salt and Mesozoic depositional trends, Powder River basin, Wyoming

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. 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 the latest Jurassic and Early Cretaceous. Many producing fields from the Mowry, Muddy, and Dakota formations exhibit either rapid stratigraphic changes syndepositional to salt collapsemore » 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.« less