Mixed Carbonate-Siliciclastic Ramp Signature of Transitional Eustasy, Mississippian Big Lime, West Virginia, U.S.A.

Thomas C. Wynn¹ and J. Fred Read^2
1 Lock Haven University, Lock Haven, PA
² Virginia Tech, Blacksburg, VA

Well-cuttings and wireline logs from 200 relatively shallow wells (0-4000 ft.) throughout West Virginia were analysed utilizing GIS. The coarse fraction (1-2 mm) of the cuttings for each sample interval was washed, grouped into Dunham lithofacies types, counted and plotted against depth. Cuttings logs were slipped (typically 10 ft or so) for the best match with digitized wireline logs to account for drilling lag, and an interpreted lithologic log was produced. Fifteen fourth order sequences (typically 20 to 150 ft thick, few hundred thousand year duration) were traced throughout the subsurface. Lowstand-early transgressive tracts on the ramp consist of a complex mosaic of redbeds adjacent tectonic highlands, barrier sands along the ramp margin, and lagoonal shales and quartz peloid grainstone eolianites. Highstand tracts consist of widespread lagoonal carbonate mudstone, interspersed with three variably developed ooid and skeletal grainstone belts located along the ramp margin, within the lagoon, and along the updip shoreline; the ramp margin facies pass downdip into dark skeletal wackestone and then into dark gray laminated argillaceous carbonates. Complex distribution of grainstone facies and re-entrants in the margin suggest a subtle 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. The GIS time slice maps provide the first view of potential Big Lime reservoir trends and associated facies throughout the state.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005