SOURCE ROCK QUALITY AND MATURATION TRENDS IN THE UTICA SHALE PLAY IN EAST-CENTRAL OHIO

A recent evaluation of the Utica Shale play by a consortium of geoscientists has shown it to contain significant amounts of liquid and gas petroleum resources. The Upper Ordovician Utica Shale play in the Northern Appalachian Basin includes, in stratigraphically descending order, the Kope, Utica, Point Pleasant, and Upper Trenton/Lexington Limestone Formations, and Logana and Curdsville Limestone Members. To assess the petroleum potential of the Utica Shale play, samples were analyzed for total organic carbon content (TOC), and bitumen reflectance (BRo random). Additional TOC and Rock-Eval pyrolysis data, from existing datasets, were also incorporated into the study.

TOC results were found to be highly variable, both spatially and stratigraphically, with values from <1 to >5 % being observed. Much of this variation appears to be the result of fine-scale organic-rich shale, and organic-poor limestone interlamination. In general, however, the Utica and Point Pleasant Formations, and Logana Member tended to have the highest TOC. Previously collected results for Rock-Eval analyses of Upper Ordovician strata were also analyzed. Examination of these data revealed a high degree of variability in the pyrolysis results, both between adjacent wells, and even within the same unit of individual wells. The variability is similar to the TOC results, and appears to be a reflection of weak, or poorly defined, S2 peaks on the pyrolysis curves.

Petrographically, most of the organic material in the Utica shale play samples occurs as amorphous organic matter (AOM), with varying amounts of bitumen. The level of thermal maturity in the Utica play, based on bitumen reflectance values (BRo), shows a progression of increasing bitumen reflectance from west to east, with a very steep increase occurring in eastern Ohio. BRo random values from central Ohio ranged from 0.66 to 0.84%, where sample depths were between 100 and 4800 ft. In eastern Ohio, sample depths ranged from 8700 ft. to more than 15,000 ft., and corresponding BRo random values ranged from 0.94 to 1.43%. Thermal maturity analyses of bitumen reflectance broadly matched expected maturities based upon the fluid content (oil, wet gas, dry gas, etc.) from initial production reports in the eastern Ohio Utica Play area, and also with Tmax values from the Rock-Eval analyses.