SUBSURFACE HIGH RESOLUTION SEQUENCE STRATIGRAPHY VIA WELL-CUTTINGS, MISSISSIPPIAN CARBONATES, APPALACHIAN BASIN

The thick mixed carbonate-siliciclastic Mississippian Greenbrier Group, West Virginia, formed on the western and northern distal margins of the Appalachian foreland basin. They thicken from less than 50m in the north and west to up to 500m in the foredeep. A high resolution sequence stratigraphy has been completed for the outcrop belts, but prior to this study, little detailed regional work had been done on the subsurface. High-resolution sequence stratigraphy of the subsurface Greenbrier Group using well-cuttings (200 wells) and wireline logs is being done throughout West Virginia. Detailed regional cross-sections and isopachs of time slices are being prepared to understand subtle depositional and structural topographic influences on facies distribution and sequence development. There are five major sequences, each composed of two to five regionally mappable high frequency sequences (HFS’s). Major- and high frequency sequences are composed of lowstand red beds up-dip and shallow marine sands along the ramp margin, semi-transgressive shale’s, 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 in the wells are placed below lowstand sands, red beds, or eolianites and through sample intervals with caliche. Cross-sections show a subtle regional, linear tectonic high along the ramp margin near the basinal hinge line, separating the relatively stable up-dip sections from the faster subsiding basin. Additionally, rapid thickening of units occurs across tilted fault blocks on the foreland. Subsidence rates in the up-dip and downdip areas differ by an order of magnitude, yet the eustatic signal, which formed the 3rd order sequences and the HFS’s is traceable from the Appalachian Basin into the Illinois Basin. Understanding the sequence stratigraphic development of Carboniferous platforms has become extremely important as a result of the super-giant petroleum reservoirs (Tengiz and Korolev) that have been found recently in carbonate rocks of this age in the Caspian basin and surrounding areas.