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Geometric Hierarchy of Sandstone Bodies in the Shannon Sandstone Member Near the Northern Black Hills, Montana and South Dakota
Natural gas has recently been produced from the Shannon Sandstone Member of the Gammon Shale (Upper Cretaceous) at shallow depths on the southwest flank of the Williston Basin. In outcrops on the north flank of the Black Hills Uplift in Montana and South Dakota, the sandstone is known as the Groat Sandstone Bed in the Gammon Member of the Pierre Shale, but the producing interval is contiguous with the Shannon Sandstone Member of the Steele Shale in the central and western Power River Basin. Paleogeographic reconstructions show that the sandstone in the northern Black Hills area was deposited on a broad shelf more than 322 km (200 mi) from the shoreline. Regional subsurface studies show that the sandstone lithosome consists of eight regional lentils, four of which coalesce laterally through the central part of the lithosome.
Exposures of the Groat Sandstone Bed in the Groat Creek area indicate that a series of four lithologic units form a coarsening-upward cycle within one of the regional lentils. The lentil is made up of at least two elongated lenses of sandstone documented in detailed subsurface studies adjacent to the outcrop belt. In outcrop, elongated lenses of sandstone are in turn composed of small-scale facies packages composed of crossbedded, burrowed, and structureless sandstone; the packages are arranged in imbricate fashion.
The lithosome, regional lentils, elongated lenses, and small-scale facies packages constitute a geometric hierarchy. The hierarchy is an ancient response model which is similar to a hierarchy of morphologic elements observed on modern continental shelves. Comparison with the modern hierarchical response model suggests that regional lentils may be interpreted as retreat massifs or sand-ridge fields, the elongated lenses as sand ridges, and the small-scale facies packages as sand waves.
Low-porosity and low-permeability reservoir rocks are found throughout the larger elements of the geometric hierarchy, such as the total lithosome. Reservoirs with higher porosity and permeability are mainly confined to sandstone bodies displaying the geometries of the smaller hierarchical elements, such as elongated lenses.
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