Thermogenic Gas Resources in the Marcellus, Utica, and Lockatong Shales – A Comparative Assessment of Devonian, Ordovician, and Triassic Unconventional Reservoirs in Pennsylvania

Christopher D. Laughrey
Pennsylvania Geological Survey, Pittsburgh, PA 15222, [email protected]

The Marcellus Shale in Pennsylvania is coming of age as one of the most significant thermogenic shale-gas systems in North America. TOC ranges from 2 to over 10 weight percent. The original kerogen consisted mostly of type II marine organic matter. Commercial Marcellus fields occur where the kerogens are thermally late mature to post mature. The produced hydrocarbons range from wet oilassociated gases to nonassociated dry gases. Carbon isotopic reversals in the latter gases indicate the breakdown of higher hydrocarbons at high temperatures during deep burial. The total prospective area in the basin that may be economic exceeds 15 million acres. Published estimates of recoverable gas range from 1,925 Bcf to 262 Tcf.

Industry interest in thermogenic shale-gas and success in defining the Marcellus resource has spurred exploration for new reserves in other shale-gas systems in Pennsylvania – the Ordovician Utica Shale and the Triassic Lockatong Formation. TOC in the Utica Shale ranges from less than 1.0 to 3.74 weight percent. The original kerogens consisted of Type II organic matter. Conodont alteration indices and Rock-Eval pyrolysis data indicate the Utica is thermally mature in northwestern Pennsylvania and post mature throughout the rest of the state. Hydrocarbons sampled from deep wells in Washington and Tioga counties are nonassociated dry gases. These Utica gases exhibit marked carbon and hydrogen isotopic reversals, a trend previously documented in deep Appalachian gases and attributed to reduction-oxidation reactions at temperatures of 200° to 300° C during maximum burial. The thickest sections of post mature Utica Shale occur in central and eastern Pennsylvania and in eastern New York State where the rocks are most prospective for thermogenic shale-gas.

Two wells tested the Lockatong Formation. Encouraging gas shows were encountered in fractured lacustrine shales between 2900 and 6200 ft. (884 and 1890 m) in the Cabot- KBI well, but the reservoir was damaged during drilling. TOC is variable in the well, but generally quite low. The kerogens originally consisted of Type II organic matter and are post mature. There are no residual liquid hydrocarbons remaining in the rocks and the kerogens have no additional generating potential. Regional sampling and geochemical analyses of the Lockatong shales in the Pennsylvania portion of the Newark basin yielded similar results. The uneven distribution of organic-rich shales and rapid variations observed in thermal maturity across short intervals make exploration for hydrocarbons difficult and speculative at best. Nevertheless, several new exploratory wells are permitted and slated for drilling.

AAPG Search and Discovery Article #90095©2009 AAPG Eastern Section Meeting, Evansville, Indiana, September 20-22, 2009