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
NUCCIO, VITO F., WANDREY, CRAIG J., U.S. Geological Survey, Denver, CO; RYDER, ROBERT T., and HARRIS, ANITA G., U.S. Geological Survey, Reston, VA
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in the central Appalachian basin, was constructed from published and unpublished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 °F (99 °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 °F (154 °C), and generated petroleum between 300-180 Ma.
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