Petroleum Geochemistry of Postmature Source Rocks in the Trenton and Black River Groups of Central Pennsylvania – An Unusual Potential Gas Source?

Laughrey, Christopher D.
Pennsylvania Department of Conservation and Natural Resources, Topographic and Geologic Survey, Pittsburgh, PA

The Ordovician Antes Shale and some carbonates and shales of the Trenton and Black River Groups of central Pennsylvania are spent petroleum source rocks. Total organic carbon in these rocks ranges from 0.35 to 3.74 weight percent. The organic matter in the rocks consists predominately of non-structured kerogens with thermal alteration indices of 4.0 to 5.0. Thermal maturation indicators such as conodont alteration index, T_max and production index confirm that these Ordovician source rocks are postmature. In some deeps wells in central Pennsylvania, however, solvent extracts and Rock-Eval pyrolysis reveals that the Trenton and Black River Groups contain surprisingly large quantities of resident hydrocarbons (> 1000 ppm) despite the high thermal maturity of the organic matter found in the rocks. Thermal extract gas chromatography indicates that these hydrocarbons are natural assemblages of mostly nC₁₀ to nC₂₀ alkanes and low-molecular weight aromatics. In some wells, natural gas shows correlate with these intervals that contain resident hydrocarbons, suggesting that the postmature Trenton and Black River sections still might have effective source rocks for commercial gas accumulations.

Further analyses of these resident hydrocarbon assemblages reveal that most of the free hydrocarbons (S₁) and pyrolytic hydrocarbons (S₂) are removed from the rocks by Soxhlet extraction. This indicates that the generative capacity of kerogens in the Trenton and Black River rocks of central Pennsylvania is indeed exhausted. All of the S₁ and S₂ hydrocarbons recovered during Rock-Eval pyrolysis occur as bitumen. These hydrocarbons probably are adsorbed on the mineral matrix or in the organic carbon. This sorption might have stabilized the hydrocarbons and allowed them to survive thermal cracking.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004