Organic geochemistry of Upper Carboniferous bituminous coals and clastic sediments from the Lublin Coal Basin (Poland)

Marek R. Gola, Michał Karger, Lucjan Gazda, Oliwia Grafka

Abstract


Bituminous coals and clastic rocks from the Lublin Formation (Pennsylvanian, Westphalian B) were subjected to detailed biomarker and Rock-Eval analyses. The investigation of aliphatic and aromatic fractions and Rock-Eval Tmax suggests that the Carboniferous deposits attained relatively low levels of thermal maturity, at the end of the microbial processes/initial phase of the oil window. Somewhat higher values of maturity in the clastic sediments were caused by postdiagenetic biodegradation of organic matter.

The dominance of the odd carbon-numbered n-alkanes in the range n-C25 to n-C31, high concentrations of moretanes and a predominance of C28 and C29 steranes are indicative of a terrigenous origin of the organic matter in the study material. This is supported by the presence of eudesmane, bisabolane, dihydro-ar-curcumene and cadalene, found mainly in the coal samples. In addition, tri- and tetracyclic diterpanes, e. g. 16β(H)-kaurane, 16β(H)-phyllocladane,
16α(H)-kaurane and norisopimarane, were identified, suggesting an admixture of conifer ancestors among the deposited higher plants. Parameters Pr/n-C17 and R dit in the coal samples show deposition of organic matter from peat swamp environments, with the water levels varying from high (water-logged swamp) to very low (ephemeral swamp). Clastic deposits were accumulated in a flood plain environment with local small ponds/lakes. In pond/lake sediments, apart from the dominant terrigenous organic matter, research also revealed a certain quantity of algal matter, indicated, i.a.,
by the presence of tricyclic triterpanes C28 and C29 and elevated concentrations of steranes.

The Paq parameter can prove to be a useful tool in the identification of organic matter, but the processes of organic matter biodegradation observed in clastic rocks most likely influence the value of the parameter, at the same time lowering the interpretation potential of these compounds.

The value of Pr/Ph varies from 0.93 to 5.24 and from 3.49 to 22.57 in the clastic sediments and coals respectively. The microbial degradation of organic matter in both type of rocks and during early stages of diagenesis is confirmed by a high concentration of hopanes, the presence of drimane homologues, bicyclic alkanes and benzohopanes. Moreover, bacteria could also have been connected with the primary input of organic matter, which is shown by the presence of e.g. C30 neohop-13(18)-ene.


Keywords


Lublin Coal Basin; Bituminous coal; Gas chromatography-mass spectrometry; Biomarkers; Terrigenous organic matter.

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