BURIAL/THERMAL HISTORY AND HYDROCARBON GENERATION MODEL OF A CROSS SECTION THROUGH THE NORTH-CENTRAL APPALACHIAN BASIN, OHIO AND WEST VIRGINIA

We have developed a preliminary burial/thermal history and hydrocarbon generation model for a regional-scale (375 km) geologic cross section through the undeformed portion of the central Appalachian basin, from the Allegheny structural front in WV northwestward across the Rome trough to the Findlay arch in northwestern OH. The model incorporates sedimentation, compaction, uplift, and erosion, and assumes a constant basement heat flow of 60 mW/m2 (milliwatts/square meter). Relatively low thermal conductivities are assigned to Pennsylvanian coals (0.2-0.6 W/mºC or watts/meter/degree Celcius) and to Devonian kerogen-rich shales (0.8–1.2 W/mºC). Thermal maturity measurements of conodont color alteration index (CAI) and vitrinite reflectance (Ro%) from Ordovician, Devonian, and Pennsylvanian rocks constrain the model. Even with the low thermal conductivities, the model requires deposition of Permian-early Triassic sediment (subsequently removed by erosion) in order to match measured thermal maturities. We assumed that maximum burial occurred during the early Triassic with deposition of a wedge of sediment ~6000 ft thick at the southeast end of the section, thinning northwestward to ~1100 ft.

The modeling focuses on the Ordovician Utica Shale, a 100-200 ft thick, laterally extensive, hydrocarbon source rock believed to have generated the petroleum trapped in Cambrian/Ordovician carbonate and sandstone reservoirs and in Lower Silurian sandstone reservoirs. Model results are consistent with this hypothesis. The halite/anhydrite bearing Salina Group provides a seal for this petroleum system. Kerogen in the Utica Shale was assumed to be a mixture of Type II and III, and to have produced predominantly gas and lesser amounts of oil. In the Rome trough the Utica Shale began generating hydrocarbons near the end of the Early Devonian; the zone of generation did not expand beyond the western margin of the trough until Pennsylvanian time. To the northwest on the flank of the Findlay arch, the Utica Shale did not reach sufficient maturity to generate hydrocarbons until deposition of Permian strata, and over the arch crest the Utica is still immature. We investigate the possibility that some of the oil within this petroleum system originated as gas and subsequently condensed to liquid during uplift and(or) updip migration to shallower depths.