REASSESSING HYDROCARBON RESOURCES OF DEVONIAN ORGANIC-RICH SHALES IN EASTERN OHIO

The Upper Devonian siliciclastic succession of the Appalachian Basin contains several organic-rich, hydrocarbon-bearing shale units, which were studied extensively from the 1970s to the 1990s to assess gas resource potential. In more recent years, the advent of unconventional drilling technology and the potential for CO₂ sequestration in organic-rich rocks has spurred further research on these units, resulting in a large quantity of source rock analyses and additional well logs. We utilized these available data, combined with digital geophysical logs, to assess the volume of hydrocarbon resources contained in these shale units within Ohio at higher geographic and stratigraphic resolution than has previously been done.

The target units for this study were the Rhinestreet Member of the West Falls Formation, and the lower portion of the Huron Member of the Ohio Shale. Over 800 well logs were used to map the thickness and extent of these units across eastern Ohio. A total of 175 well logs had adequate digital gamma-ray and bulk density logs to use for resource estimate calculations. Thermal maturity was mapped as calculated vitrinite reflectance based on programed pyrolysis (T_max) performed on either core samples or well cuttings from 65 wells. The resulting thermal maturity maps estimate that both units are mostly within the oil window, but the western portion of the lower Huron is immature. Total organic carbon (TOC) data measured from cores were sparce, so this limited data set was used to determine the relationship between measured TOC, gamma-ray, and density log responses. The resulting equation (R² = 0.65) was applied to 175 digital logs to estimate TOC for each unit. Oil volumes were then calculated using the thermal maturity, TOC estimates, porosity, and unit thickness. General trends are different in each unit. Estimated oil-in-place is greater in the deeper, more thermally mature portions of the Rhinestreet, and greater in the shallower, more immature portions of the lower Huron. As our energy needs change and opportunities for CO₂ sequestration, enhanced oil recovery, and wastewater disposal are explored in Ohio, these higher-resolution maps and hydrocarbon estimates could be valuable tools for furthering our understanding of resource distribution throughout the state.