Geochemistry of Petroleum Source Rocks and Natural Gases at Council Run Field, Centre and Clinton Counties, Pennsylvania
Billman, Dan A.1 and Christopher D. Laughrey2
1 Billman Geologic Consultants, Inc., Mars, Pa
2 Pennsylvania Department of Conservation and Natural Resources, Topographic and Geologic Survey, Pittsburgh, PA
The Council Run gas field of north central Pennsylvania has some of the most productive Devonian sandstone reservoirs in the Appalachian basin. The field is enigmatic because of its position near the eastern edge of the Appalachian Plateau, where potential source rocks are postmature and sandstones with reservoir potential elsewhere have low porosities and permeabilities or are poorly sealed.
Potential source rocks for gas produced at Council Run include the Burket Member of the Upper Devonian Harrell Formation and the Middle Devonian Marcellus Formation. Vitrinite reflectance, thermal alteration indices and Rock-Eval pyrolysis confirm that these shales have poor petroleum potential due to advanced thermal maturation. Bitumen extracts display an odd-carbon predominance of midchain n-alkanes, and a relatively low amount of higher molecular weight compounds. These characteristics along with carbon preference indices, pristine/phytane ratios, and isoprenoid-to-n-alkane ratios also reflect significant maturation effects.
Crossplots of 13C versus D suggest that the methane produced at Council Run is thermogenic and was generated with oil. The distributions of 13C in methane, ethane, and propane suggest a mixture of 1) thermogenic gases generated in the oil window before maximum burial of the Paleozoic rocks of the region and 2) gases generated by the conversion of oil to gas during deeper burial.
We interpret the critical moment at Council Run field as having occurred between 260 and 240 Ma when most of the oil in the petroleum system was cracked to gas. Preservation time has been relatively long and was controlled by the rapid rate of uplift and denudation that occurred during Permian time as well as during Mesozoic extension.
AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004