EUSTASY AND TECTONICS OF A MISSISSIPPIAN CARBONATE RAMP, WEST VIRGINIA, USA

The 50 to 500m thick mixed carbonate-siliciclastic Mississippian Greenbrier Group of West Virginia, which formed on the Appalachian foreland, provides an outcrop and subsurface analog to better understand Mississippian reservoir stacking patterns and the stratigraphic signatures of global greenhouse to icehouse conditions. We are generating a high-resolution sequence stratigraphic framework for the Greenbrier Group throughout the subsurface in West Virginia, using data from 200 wells. A series of detailed regional cross-sections and isopachs of time slices are being prepared to better understand the transition from greenhouse to global ice-house conditions and what effect this had on reservoir development. Five major sequences occur composed of three to four regionally mappable high frequency sequences. Major- and high frequency sequences are composed of lowstand red beds up-dip and shallow marine sands along the ramp margin, semi-transgressive shales, high-stand quartz peloidal grainstone (dominantly eolian), peritidal lime mudstone, peloid grainstone, ooid grainstone, skeletal grainstone, open marine skeletal wackestone/mudstone, and shaly slope mudstone. Sequence boundaries are placed below lowstand sands, red beds, caliches and/or eolianites. Thickness trends strongly reflect tectonically induced basinal subsidence. The ramp margin in the vicinity of the basinal hinge line separates the relatively stable up-dip sections from the faster subsiding basin. It localized high-energy grainstone trends, as did subtle tectonic highs. Subsidence rates in the up-dip and down dip areas differ by an order of magnitude, yet the eustatic signal was the dominant influence on deposition of the 3rd order sequences and component 4th order sequences. The extremely complex sequence stratigraphy on the platform, suggests a complex history of relative sea level change. This complex sea-level history has caused the rapid lateral facies heterogeneity on the platform. Three-D mapping of the sequence stratigraphic time slices showing the thickness variation and distribution of environmentally sensitive facies is the best way to define the complex influence of tectonics and eustasy and their effects on the stacking patterns of reservoirs on the platforms.