PETROGRAPHY AND GEOCHEMISTRY OF THE DEVONIAN OHIO SHALE FROM A DRILL CORE IN EASTERN KENTUCKY: IMPLICATIONS FOR POTENTIAL CO2 SEQUESTRATION AND ENHANCED METHANE PRODUCTION

Nearly two-thirds of Kentucky is underlain by organic-rich shales of Middle – Late Devonian age. In eastern Kentucky, the shale thickens from less than 30 ft in the central part of Kentucky to more than 1600 ft in areas to the southeast. The shales are a significant source of natural gas in Kentucky, and the east-central U.S. Collectively, the shales also represent a potentially large area for carbon capture and storage, as well as enhanced methane production. In the Appalachian Basin alone, estimates indicate that approximately 247 Gt of CO₂ could be sequestered in the Devonian shale.

Devonian age Ohio shale samples from the Rosewood 02 Bargo well, located in Knox County, Kentucky, were analyzed geochemically and petrographically. Specifically, this was done to develop a better understanding between the composition and distribution of organic material in the shale, and CH₄/CO₂ gas adsorption affinity. Devonian shales from the Bargo well are all very fine-grained, fissile, and organic rich. X-ray diffraction analyses indicate that the samples average 40.6 % clay minerals, 34.6 % quartz, 5.1 % carbonate minerals, and 13.7 % pyrite. Geochemically, the samples range in total organic carbon (TOC) from 0.5 to 10.7 % (avg. 5.0 %), with the majority being Type II (oil prone) or mixed type II/III (oil + gas prone) kerogen. Petrographically, all of the samples are dominated by matrix bituminite, which is mainly observed as a coating on most of the shale matrix. Other macerals are minor in abundance by comparison. Vitrinite reflectance (R_o ) measurements taken from across the entire section (120 ft) range from 0.76 to 0.83 (avg. 0.79), indicating a level of thermal maturity sufficient for the generation of oil and gas.

CH₄/CO₂ adsorption isotherm analyses, performed on four composite intervals (based on similar total organic carbon contents) from the Bargo well, indicate an average adsorption capacity of 102 scf/ton for CO₂ and 25 scf/ton for CH₄ at ~400 psia. Hence, the shales can hold approximately four times the amount of CO₂ than they can CH₄ at this pressure. In addition, permeability tests were performed on three intact sidewall core samples. Overall low matrix permeabilities were low, indicating that fracture permeability (natural and enhanced) will be critical to the future of any sequestration/ enhanced recovery of gases in the shale.