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Thermal Maturity Patterns for Ordovician, Devonian, and Pennsylvanian Rocks of the Central Appalachian Basin, Based on CAI and Vitrinite Reflectance (%Ro)

Repetski, John E., Leslie F. Ruppert, Robert T. Ryder, David J. Weary, Elisabeth L. Rowan, and Michael H. Trippi
U.S. Geological Survey, Reston, VA

New thermal maturation maps of the central Appalachian basin are presented for Ordovician and Devonian rocks, using conodont color alteration index (CAI) and vitrinite reflectance (%Ro) values derived from drill cuttings in ~300 wells, and for Pennsylvanian rocks, using %Ro values from coal beds. Intervals were chosen because they are known to contain conodonts and/or are within/near probable reservoirs and source beds for Appalachian oil, gas, and coal bed methane (CBM) accumulations. Paleothermal isograds based on these new data complement, refine, and extend westward the almost wholly outcrop-based CAI patterns of earlier maps. The new maps show thermal patterns aligned with, and probably causally related to, structural features such as the Rome trough and other basement-fault systems. The Ordovician and Devonian CAI and %Ro isograds show differing levels of compatibility with known petroleum accumulations, suggesting that locally sourced accumulations may be distinguished from those originating from longer-distance migration. The 0.6 to 0.8 %Ro isograds help define the approximate lower limits of thermogenic gas generation in Devonian shale and Pennsylvanian coal beds.

The CAI and %Ro isograds indicate significantly greater overburden than present today over most of the region. Some of the higher isograds likely require additional heating, possibly related to crustal thinning/emplacement of mantle-derived rocks along reactivated extensional structures in post-Paleozoic time, migration of geothermal/hydrothermal fluids, and insulation of lower Paleozoic rocks by Devonian black shale and Pennsylvanian coal. Burial/thermal history, hydrocarbon generation, and fluid flow models are being developed to test the plausibility of some of these ideas.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004