Evaluating clay mineralogy as a thermal maturity indicator for Upper Devonian black and grey shales and siltstones within the Ohio Appalachian Basin

The clay mineralogy of the Upper Devonian age Chagrin and Huron Shale Members of the Ohio Shale was analyzed over in eastern Ohio to evaluate the usefulness of comparing the ratio between illite and smectite within mixed layer clay minerals to estimate thermal maturity. The process by which smectite diagenetically converts over time to illite via burial diagenesis has been rigorously studied in the past because of its relationship with thermal maturity. While simply understanding the thermal maturity of target formations is important, studies of illitization with depth can also give insight to hydrocarbon generation windows. This relationship allows for a prediction to be made on the stages of hydrocarbon generation that have occurred in a target formation based on the ratio of illite to smectite and the Reichweite ordering within the mixed layer clay.;Eighty-four samples were taken across an 8 county study area from the top and center of the Chagrin Shale Member and the center of the Huron Shale Member. X-ray diffraction (XRD) analysis was carried out on both bulk powder and clay fraction powder samples to gain an understanding of both clay and nonclay minerals present in each sample. Clay fraction samples were analyzed in the air-dried state, after treatment with ethylene glycol, and after heating to 550° for 30 minutes, to identify all clays present. The Kubler index, the measure of full width at half maximum (FWHM), of the 10 A illite peak in the air-dried state was also recorded using XRD analysis for each sample. This was done to help identify the presence of mixed layer clays containing only a small expandable component. Environmental Scanning Electron Microscope (ESEM) imaging was carried out to evaluate clay authigenesis with depth, and total organic carbon (TO C) was measured via elemental combustion in a CHN analyzer.;XRD analysis revealed that illite is the dominant clay mineral in all of the samples analyzed in the study and, on average, illite increased in abundance with depth. Reichweite ordering values within mixed layer clays, as interpreted from changes between diffraction patterns in the air-dried state and after treatment with ethylene glycol, were R3 ordered on average. Furthermore, the Kubler indices of the illite peak for all but two samples analyzed lie between 0.4 and 0.65, which corresponds to thermal conditions promoting hydrocarbon generation. ESEM images confirm the increase in authigenic illite with depth. However, this finding is more qualitative than quantitative because of the small size of clay mineral growth in shale pore spaces. The general increase of illite abundance with depth, the level of Reichweite ordering within the mixed layer clays encountered, and the Kubler indices measured from the illite peak, lead to the conclusion that evaluation of clay mineralogy to estimate thermal maturity is more accurate than vitrinite reflectance in the Upper Devonian shales of Ohio. Vitrinite reflectance can be suppressed by high abundances of alginitic organic matter (Laughrey, 2012). It is also recommended that the immaure/oil window boundary from previous studies (Milici and Swezey, 2006; Rowan, 2006) be moved far enough west to include the study area.