A Comparison of Geochemical Features of Extracts from Coal-Seams Source Rocks with Different Polarity Solvents

Abstract

There exists a great difference in group-type fractions and biomarker distributions of chloroform extracts from coals and coal-seams oils, which makes the source identification of coal-seams oils in sedimentary basins rather difficult. The experiment, in which four different polarity solvents, n-hexane, benzene, dichloromethane and chloroform, were used to extract 9 coal-seams source rocks and 3 typical lacustrine source rocks, showed that the yield of extracts increased gradually with increasing solvent polarity, among which obviously the proportion of n-alkanes gradually decreased and the composition of the n-hexane extract was the most similar to that of coal-formed oils. Although some obvious differences existed in yields and compositions of the extracts obtained by different solvents, the entire distribution features of their n-alkanes, isoprenoids and sterane and terpane biomarkers remained, in general, similar, showing no distinct enrichment or depletion for a certain fraction by any solvent. The compositional analysis on n-hexane and chloroform extracts showed that the absolute amount (concentration) of biomarkers was relatively low for the n-hexane extract but comparatively high for the chloroform extract, this difference became great among coal-seams source rocks but small among lacustrine mudstones. The statistical analysis on the relative amount of the 18 major biomarkers in n-hexane and chloroform extracts from 10 source rock samples showed that extracts with a proportional error for the same biomarker of less than 5% (including the analytical error) accounted for 84% while those with a proportional error over 10% amounted to below 5%. This suggested that the difference in solvent polarity would lead to variations in both the relative amount of n-alkanes and the absolute amount of biomarkers for the extract from coals but could not induce variations in the distributional features and mutual ratios of biomarkers for the same extract on condition of using common solvents, consequently, the outcome of oil-source correlation made by these biomarkers will be independent of variations in amounts of saturates and biomarkers arising from solvent polarity. Therefore, biomarkers obtained from organic-rich source rocks including coals by the extraction with the commonly used chloroform solvent can be applied for the oil-source correlation of coal-seams petroliferous basins.