Significant concentrations of hydrogen sulphide (H2S) are observed in the Marrat reservoir in the Raudhatain and Sabriya fields (northern Kuwait). In this study, we have attempted to determine the origin of H2S and what factors may control its relative concentration. Our data show that it is most likely a product of thermochemical sulphate reduction (TSR). However, this process has not gone to completion, as the levels of H2S in the samples studied are still below 5%. Additionally, the hydrocarbon constituents of the gas phase do not appear to have been severely affected by TSR. So far, this process has primarily impacted the oil phase: increasing oil gravity and decreasing oil sulphur content. Some of the Marrat fluids from the Sabriyah field have lower H2S concentrations, though with equivalent H2S sulphur isotopic compositions to the other Marrat fluids from Raudhatain. We hypothesize that these differences are related to original compositional grading and/or reservoir charge histories, which influence the degree of TSR alteration by virtue of differences in original fluid polar and associated labile sulphur contents. This study demonstrates that pre-TSR reservoir charge histories and fluid phase distributions should be taken in to account for effective evaluation of H2S occurrences.


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