Fluid inclusion evidence for low-temperature thermochemical sulfate reduction (TSR) of dry coal gas in Upper Permian carbonate reservoirs (Zechstein, Ca2) in the North German Basin

摘要

Upper Permian Zechstein carbonate Ca2 gas reservoirs in the southern part of the Pompeckj Block in the North German Basin locally contain up to 36 vol% hydrogen sulfide (H2S) produced by thermochemical sulfate reduction (TSR). TSR was triggered by migration of dry to extremely dry coal gas from Upper Carboniferous into the Zechstein carbonate reservoirs. Methane reacted with dissolved sulfate at temperatures of < 150 degrees C, as inferred from fluid inclusions hosted in fracture-filling minerals and cements in the carbonate reservoir rocks. Such low temperatures for methane-dominated TSR are unique and were not observed so far, as it was widely believed that alteration of super dry methane requires much higher temperatures. Here we present detailed compositional and carbon isotope data of reservoir gases as well as those of gases trapped in fluid inclusions hosted in cements and fracture-filling minerals in Zechstein Ca2 carbonate reservoir rocks. We constrained the P-T conditions of gas entrapment, hydrocarbon reactivity and the lower temperature limit for TSR.