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Hydrocarbons Sourced from the Mississippian-Devonian Black Shales in the Appalachian and Illinois Basins *
By
Patrick J. Gooding1 and Frank R. Ettensohn2
Search and Discovery Article #10152 (2008)
Posted October 22, 2008
*Adapted from oral presentation at AAPG Annual
Convention,
1Kentucky Geological Survey, University of Kentucky, Lexington, KY ([email protected])
2Department of Earth and Environmental Sciences, University of Kentucky, Lexington, KY
On the west flank of the Appalachian Basin, most of the Middle Devonian and Lower Mississippian section is characterized by fine-grained clastic rocks, about 75 percent black shale. The remaining stratigraphic sequence is divided nearly equally between light-colored Lower Mississippian shales in the uppermost part and Middle Devonian dolostones in the lowermost part. The shales attained thickness of over 2000 feet in the Appalachian Basin, about 500 feet in the Illinois Basin and less than 100 feet across the Cincinnati Arch. The black shales represent the distal, or basinal, equivalents of major deltaic progradations in the Catskill delta complex on the eastern side of the Appalachian Basin. These major progradations were preceded or accompanied by episodes of subsidence in the Appalachian peripheral basin and in intracratonic basins like the Illinois Basin. The subsidence and progradations can be clearly related to tectophases in the Acadian Orogeny. Tectonic and paleogeographic constraints kept most of the coarse-grained clastic materials in the eastern, or proximal, parts of the Appalachian Basin so that distal, or western, parts of the basin and parts of adjacent cratonic seas were dominated by very fine-grained clastic sediments and abundant organic matter. The abundance of organic matter in these shales has been related to paleoclimatic and paleogeographic conditions unique to Devonian time, as well as to the enhanced preservation potential accompanying development of a stratified water column during periods of subsidence. Post-depositional down warping during the Carboniferous and Permian provided sufficient burial to attain thermal maturity and generate hydrocarbons. Further warping associated with the Appalachian Orogeny created the folds and faults, avenues of transport and trapping, through which the generated hydrocarbons could migrate.
