3Pennsylvania Geological Survey, Pittsburgh, PA 15222

Conodont color alteration index (CAI) isograds of 4, 4.5, and 5 in Ordovician rocks define a region of high thermal maturation (200-mi-long by 140-mi-wide) that extends from north-central/northeastern Pennsylvania into south-central/southeastern New York. The broad central part of the thermal maturity high is marked by CAI 4.5 and 5 values, whereas the northern and western margins of the high are defined by closely spaced CAI 3.5, 4, and 4.5 isograds. CAI 2.5 and 3 isograds in Lower-Middle Devonian rocks of south-central New York closely mimic the thermal maturation pattern defined by the CAI 3.5-4.5 isograds in the underlying Ordovician rocks. Previously recognized variations in illite/smectite and apatite fission-track ages in a Middle Devonian bentonite bed corroborate the western margin of the thermal maturity high.

We interpret the thermal maturity high to represent the original outline of the northeastern part of the Appalachian foreland basin where a thick overburden of Upper Devonian Catskill delta and Carboniferous deposits had accumulated. Although this overburden is now absent, it may have been as thick as 20,000 ft according to one estimate. A secondary factor that may have contributed to the thermal maturity high is higher-than-normal heat flow caused by crustal extension, intrusion of late Mesozoic kimberlite bodies, or warm circulating fluids.

Natural gas accumulations in Ordovician and Silurian reservoirs within the thermal maturity high are dry (C₁/C₁-C₅ ≥ 0.98) and isotopically heavy (δ¹³C_methane ≈ -31 per mil). These gases probably were derived from the Ordovician Utica Shale source rock and commonly involved vertical migration of about 1,000 ft.

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York