OUTCROP ANALOGS FOR SUBSURFACE MISSISSIPPIAN GREENBRIER LIMESTONE RESERVOIRS, WEST VIRGINIA, USA

The Mississippian Greenbrier Group (“Big Lime”), West Virginia, is a 100 to 1600 feet thick succession of mixed carbonate-siliciclastic sediments that formed on the Appalachian foreland. Exposures provide outcrop analogs to better understand subsurface Mississippian 3-D facies distribution, reservoir stacking patterns, and the sequence stratigraphic signature of global greenhouse to icehouse conditions within an active foreland setting. In the past the Greenbrier Group was lumped into large litho-stratigraphic units. This rock-stratigraphic approach did not allow the facies distribution, stacking patterns, reservoir trends, and effects of tectonics to be elucidated, because there was no obvious way to time-slice the intervals. Well-cuttings data and wireline logs in conjunction with limited core and outcrop data were used to generate a regional, three-dimensional high resolution sequence framework. The outcrop sections and two cores provide insight into the stacking of facies making up sequences, parasequence stacking, as well as examples of bounding surfaces, which are beyond the resolution of all but cored sections from the subsurface. Depositional sequences and their component facies were traceable from the outcrop sections into the subsurface using outcrop gamma-ray, well cuttings and wireline logs. Sequences were recognized on the basis of major landward and basinward shifts in diagnostic facies belts such as eolianites, red beds, siliciclastic sands/calcareous siltstones, and lime grainstones. Sequence boundaries were picked at erosional disconformities, on top of shallowing-upward carbonate units, and below lowstand or transgressive siliciclastic red beds, sandstones and shale. Where possible, maximum flooding surfaces were picked below the deepest marine facies within the sequence, but the subsurface data commonly prevented them being traced regionally, thus it was possible to separate the transgressive and highstand systems tracts only locally on the ramp. Thickness trends and complex distribution of grainstone facies and re-entrants in the margin suggest a tectonic control on facies distribution associated with the complexly faulted foreland, while the major sequence development reflects 4th order eustasy driven by waxing and waning of Gondwanan ice-sheets.