THERMAL MATURITY PATTERNS FOR ORDOVICIAN AND DEVONIAN ROCKS OF THE CENTRAL APPALACHIAN BASIN, BASED ON CAI AND %RO

New thermal maturation maps have been constructed for Ordovician and Devonian rocks of New York, Ohio, Pennsylvania, and West Virginia using conodont color alteration index (CAI) and vitrinite reflectance (%Ro) values derived from drill cuttings in more than 200 wells. The Ordovician and Devonian carbonate intervals were chosen because (1) they are known to be conodont bearing and (2) they are within or near target reservoir zones and probable source beds for most current and anticipated Appalachian oil and gas accumulations. The addition of these subsurface data have resulted in paleothermal isograd patterns that complement, refine, and extend westward the almost wholly outcrop-based CAI patterns of Anita Harris and colleagues in 1978. For example the new map shows thermal patterns that are aligned with, and probably causally related to, structural/geophysical features such as the Rome trough, Scranton gravity high, and selected basement-fault systems. Also, the CAI and %Ro isograds show differing levels of compatibility with known petroleum accumulations, suggesting that those accumulations originating from local migration may be distinguished from ones originating from longer-distance migration. The %Ro 0.6 isograd helps define the western limit of self-sourced gas in Devonian shale.

As in other paleothermal investigations in the Appalachians, the CAI and %Ro isograds indicate much greater thicknesses of overburden than are present today over most of the region. Even so, some of the higher isograds likely require some additional heating. Possible sources of this elevated geothermal flux are crustal thinning and emplacement of mantle-derived rocks along reactivated extensional structures in post-Paleozoic time, migration of geothermal fluids, and insulation of lower Paleozoic rocks by Devonian black shale and Carboniferous coal. Burial history/thermal history, hydrocarbon generation, and fluid flow models can test the plausibility of many of these ideas.