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Abstract: Thermal Maturity and Petroleum Generation of Middle Ordovician Black Shale Source Rocks, Central Appalachian Basin--Controls on Oil and Gas in Lower Silurian Low-Permeability Sandstone Reservoirs


A vitrinite reflectance equivalent (VRE) map of Middle Ordovician strata in the central Appalachian basin, was constructed from published and unpublished thermal maturity data (Conodont Color Alteration Index, and Rock-Eval pyrolysis) to determine regional thermal maturity patterns. Burial, thermal and petroleum generation models were also constructed to characterize the thermal history and timing of petroleum generation of Middle Ordovician black shale (Utica and Antes Shales), the likely source of regional oil and gas accumulation in the overlying Lower Silurian low-permeability sandstone reservoirs.

The distribution of oil and gas fields in the Lower Silurian "Clinton" sands, Medina Group, and Tuscarora Sandstone reservoirs does not follow VRE trends. In general, mainly oil and associated gas are produced in central and east-central Ohio, whereas mainly nonassociated gas is produced in adjoining northwestern Pennsylvania and in western New York. One would expect a VRE line of around 1.35 (oil deadline) to follow this north- to northwest trending oil-gas transition. However, the 0.75 percent line (initiation of significant oil and gas generation), the 1.10 percent line (threshold of intense gas generation), and the 1.35 percent VRE line are parallel to northeast-trending structure contours and cut at high angles across the oil-gas transition.

Petroleum generation models indicate very different thermal regimes and timing of oil and gas generation throughout the region. In central Ohio, Middle Ordovician source rocks were buried to about 8,300 ft (2,530 m) and achieved a maximum temperature of 210 degrees F (99 degrees C): too immature for significant oil and gas generation. In northern West Virginia, these source rocks were buried to about 19,000 ft (5,791 m), reached temperatures of 310 degrees F (154 degrees C), and generated petroleum between 300-180 Ma.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky