Stratigraphic Compartmentation of Reservoir Sandstones: Examples from the Muddy Sandstone, Powder River Basin, Wyoming
Randi S. Martinsen, 1994. "Stratigraphic Compartmentation of Reservoir Sandstones: Examples from the Muddy Sandstone, Powder River Basin, Wyoming", Basin Compartments and Seals, Peter J. Ortoleva
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The Lower Cretaceous Muddy Sandstone (Viking Formation equivalent) is a thin but complex stratigraphic unit that contains a variety of anomalously pressured compartments. One or more lowstand surfaces of subaerial exposure and erosion (LSEs), numerous transgressive surfaces of submarine erosion (TSEs), and varying lithofacies compartmentalize the Muddy Sandstone stratigraphically on at least three levels. The first level of compartmentation is defined by the relief along the LSE surface(s), which is highly variable and physically divides the Muddy, both vertically and laterally, into older and younger sequences. The second level is defined by the intersection of shales above the TSEs with the LSE (either by onlap or truncation). The third level results from variations in lithofacies. Whereas many of the compartments comprise classic stratigraphic traps consisting of shale (seal) encompassing sandstone (reservoir/compartment), compartments exist wherein sand is juxtaposed against sand without benefit of intervening shales to serve as a seal. In these situations, the seal appears to consist of a paleosol developed beneath the LSE. The distribution and geometries of pressure compartments in the Muddy have a high degree of correspondence to the various scales of stratigraphic compartmentation observed. In all probability, similar levels of stratigraphic complexity characterize many basins. Any analysis of the controls on pressure compartment formation and distribution therefore should incorporate these stratigraphic complexities and not assume that stratigraphic systems are characteristically simple.
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Basins worldwide exhibit an unexpected degree of hydrologic segregation. There can be regions of a sedimentary basin that are isolated from their surroundings by a relatively thin envelope of low-permeability rock with an interior of sufficiently high permeability to maintain a consistent internal hydrostatic fluid pressure gradient. These have been named pressure compartments. Presure compartments have several remarkable features, just one of which is that internal fluid pressures can greatly exceed or be significantly less than any regional topographically controlled hydrologic head or drain. This publication contains 30 chapters that take detailed looks at pressure compartments in general, and detail case studies of these compartments in specific basins, such as the Anadarko and Gulf of Mexico. The volume also looks at other considerations in sedimentary basins such as hydrodynamic and thermal characteristics, and mechanical properties of rock.