Petroleum Geochemistry


The Trenton Formation, Antes Shale, and Utica Shale are known petroleum source rocks in central and western Pennsylvania (Wallace and Roen, 1989; Ryder and others, 1998). The petroleum potential, based on total organic carbon, is good to excellent in these rocks. All of these rocks are thermally postmature, however, here in central Pennsylvania. Conodont Alteration Indices (CAI) exceed 4.0 (Repetski and others, 2002), and Thermal Alteration Indicies (TAI) range from 4 to 5. Nevertheless, pyrolysis data from exploratory wells in central Pennsylvania indicate that the Trenton and Black River Groups appear to be generating low-molecular weight hydrocarbons at a rate that might result in commercial gas accumulations. These hydrocarbons were generated earlier in the burial and thermal evolution of the now exhausted kerogens, and are sorbed on the mineral matrix and/or organic carbon in the limestones. These hydrocarbons are nC10 to nC24 alkanes, and low molecular weight aromatics with some nC9 to nC16 alkanes. The sorbing medium apparently stabilized the hydrocarbons, and allowed them to survive the thermal cracking that might otherwise have destroyed them at the observed level of thermal maturation in these rocks. Further geochemical study is needed to document the nature and origin of these sorbed hydrocarbons and their commercial potential.