In the Southern Zechstein Basin there is an exploration risk related with high nitrogen content in the Main Dolomite (Ca2) reservoir. Existing theory of nitrogen-rich gases origin in Ca2 reservoirs (the Sulecin isolated platform) states, that natural gas was generated from type III kerogen at high-temperature stage of thermogenic processes and probably migrated from the Carboniferous/Rotliegend deposits, sourced by the post-mature Carboniferous shales. A comparison of isotopic composition of gases from Ca2 and the Rotliegend reservoirs from Wedrzyn-1well does not confirm this theory. δ15N values from these reservoirs are significantly different, around +1,3‰ for the Rotliegend and +9,8‰ for Ca2 what excludes their common source. The comparison of isotopic composition of methane indicates that both gases are thermogenic. The methane from Ca2 is isotopically lighter, what suggest it contains biogenic components or was generated from organic matter of lower maturity or different composition. Nitrogen content in Ca2 gas can result from the specific organic matter type. Sedimentary conditions during deposition allowed strong development of microorganisms. Their activity during and after sedimentation might lead to nitrogen formation through biochemical reactions starting from atmospheric N2 fixation through ammonification, nitrification, ending on denitrification and sulfate reduction.


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