Organic Matter Maturation Trends and Source Rock Quality in the Utica Shale, East-Central Ohio

Abstract

The Utica Shale in east-central Ohio was recently evaluated to assess its petroleum resource potential. To achieve this, drill core and cuttings 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. In the Northern Appalachian Basin, the Upper Ordovician Utica Shale includes, in stratigraphically descending order, the Kope, Utica, Point Pleasant, and Upper Trenton/Lexington Limestone Formations, and Logana and Curdsville Limestone Members.

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. This variability is similar to the TOC results.

Petrographically, most of the organic material in the Utica shale play samples occurs as amorphous organic matter (AOM), with varying amounts of solid bitumen. Zooclasts, mainly in the form of graptolite fragments, occur frequently as well. 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 were much deeper, ranging from 8700 ft. to more than 15,000 ft. In this area, corresponding BRo random values ranged from 0.94 to 1.43%. Thermal maturity analyses of bitumen reflectance broadly match expected petroleum products based upon the fluid content (oil, wet gas, dry gas, etc.) from initial production reports. The reflectance values are also in broad agreement with calculated Ro values from Rock-Eval Tmax values.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016