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Investigation of molecular and carbon isotopic composition variability of hydrocarbon gases (methane, ethane and propane) has been performed on gases from coal and rock samples from Kozlu district of the Zonguldak Basin in order to ascertain the origin of gases and to explain possible reasons of variations in molecular and isotopic composition. For this purpose, the molecular and isotopic composition of eight headspace and associated gases from coals and adjacent organic rich sediments of two different stratigraphic levels of Kozlu formation in the Zonguldak basin have been determined. Total organic carbon (TOC) measurements and Rock-Eval pyrolysis were performed in order to determine bulk organic geochemical characteristics of the respective intervals. Coal and associated rock samples contain predominantly vitrinite rich type III organic matter with hydrogen indices (HI) up to 162 mgHC/gTOC. The stable carbon isotope ratios (δ13C) of the kerogen vary from -25.52‰ to -28.05‰. All the headspace gaseous hydrocarbons are dominated by methane (>94%). δ13C range for methane, ethane and propane of the headspace samples are defined as; δ13C1: -48.4‰ to -36.3‰, δ13C2: -26.7‰ to -17.5‰ and δ13C3: -31.8‰ to -22.3‰, respectively. Methane isotopic composition and molecular ratios of the gases show that the origin of gases is mainly thermogenic. But admixture of some biogenic gas is also possible. The amount of the biogenic admixture is controlled mainly by cleat/fracture intensity of the coal or rock. The biogenic admixture of methane increases with increasing intensity of the rock fractures. In other words, methane isotope composition becomes much lighter with increasing proximity to a fault of to a crushed zone. Isotopic composition of methane and geological history of the basin led to the conclusion, that biogenic component is related with secondary bacterial gas generation. Ethane is unexpectedly heavier than the propane in all of the analyzed samples. It becomes heavier in samples particularly from the Mileopera seam, which is intensively cleated. The gases with a negative stable carbon isotope sequence of δ13C2>δ13C3 may possibly imply a microbial alteration. Respective isotope values of headspace and associated gases can also be influenced additionally by desorption process, which is caused by uplift of the basin during the last 42 million years.

Origin and secondary alteration of coalbed and adjacent rock gases in the Zonguldak Basin, western Black Sea Turkey