Petrologic and organic geochemical characteristics of Late Permian bark coal in Mingshan coalmine, Southern China

Abstract

Bark coal is found only in Late Permian age coals, and only at some specific locations in Southern China. The depositional environment of bark coal is not yet well understood. As a contribution to improving the understanding of the origin of this unusual coal component, the petrological characteristics and organic geochemistry of bark coal were investigated by gas chromatography (GC) analysis, gas chromatography-mass spectrometry (GC-MS) analysis, and coal petrology techniques. The samples were selected from the B3 coal seam in the Mingshan coal mine, Leping area, in the south of China. The thermal maturity and organic matter type of the samples were determined by Rock-Eval analysis and from organic geochemical data. Barkinite occurs were commonly in the middle and upper parts, and less commonly in the lower parts, of the B3 coal seam. Barkinite is type II kerogen, while other samples are type II-III kerogen, consistent with good hydrocarbon-generation potential. The thermal maturity of the samples is consistent with their being of early-mature to peak oil window maturity. Gas chromatograms obtained from the samples are dominated by long chain n-alkanes (n-C22 to n-C27) and high Carbon Preference Index (CPI) values, indicating terrestrial organic matter input. The biomarker parameters obtained from mass spectrometer data on m/z 191 and m/z 217 indicate that the formation of these samples was influenced by marine environment and the deposited organic matter was preserved under relatively oxic conditions.