Organic petrology of Carbondale Formation coal beds and marine roof shales in Western Kentucky, Eastern Interior (Illinois) Basin, USA

Ten samples of Carbondale Formation (late Middle Pennsylvanian, Westphalian D) roof shale strata from the Western Kentucky Coal Field (Eastern Interior Basin, USA) were analyzed petrographically to gain a better understanding of the organic composition of the shales. The samples were collected in conjunction with a coal bed methane exploration program, so a direct comparison of the organic composition of the shales, and the coal beds that directly underlie them, could be made. Petrographically, total vitrinite contents in the shales average 20.8% (mineral matter free basis, mmf), which is significantly lower than the average vitrinite content of Carbondale Formation coal beds (avg. 81.8%, mmf; n = 32). In contrast, total liptinite contents average 49.7% (mmf) in the shales, which is significantly higher than the average liptinite content of Carbondale coals (avg. 7.0%, mmf). Amorphinite and bituminite were found to be the most abundant liptinite macerals in the shales, but were not recorded from the coal samples. Likewise, total inertinite contents were much higher in the shales (avg. 29.6%, mmf), than in the coals (avg. 11.0%, mmf). A type of macrinite identified as “granular macrinite” was the dominant inertinite maceral in the shales, but was absent in the coal samples. Although the average desorbed gas contents of Carbondale Formation coals and shales are low and currently uneconomical (avg. 0.9 ml/g [28.8 scf/ton] for the shales, 2.4 ml/g [76.9 scf/ton] for the coals, as received basis), they nonetheless represent a potential future gas resource.

► Carbondale Formation coal beds are typically overlain by organic-rich shales.
► Petrographically, Carbondale shales are different than their coal counterparts.
► A type of inertinite, termed “granular macrinite” is common in the shale samples.
► Carbondale Formation shales were found to have an average gas content of 0.9 ml/g.
► Carbondale Formation coals were found to have an average gas content of 2.4 ml/g.