Chemical and isotopic evidence for secondary alteration of natural gases in the Hetianhe Field, Bachu Uplift of the Tarim Basin

H2S and CO2 are found in elevated concentrations in the reservoirs near the Carboniferous-Ordovician unconformity in the Hetianhe Field of the Tarim Basin, NW China. Chemical and isotopic analyses have been performed on produced gases, formation waters and reservoir rocks to determine the origin of CO2 and H2S and to explain the heterogeneous distribution of isotopic and geochemical characteristics of petroleum fluids. It is unlikely that H2S and CO2 had a mantle component since associated helium has an isotope ratio totally uncharacteristic of this source. Instead, H2S and CO2 are probably the result of sulphate reduction of the light hydrocarbon gases (LHG). Increasing H2S concentrations and CO2/(CO2 + SigmaC(1-4)) values to the west of the Hetianhe Field occur commensurately with increasingly heavy hydrocarbon gas delta(13)C values. However, thermochemical sulphate reduction is unlikely because the temperatures of the reservoirs are too low, no H2S or rare pyrite was detected in deeper reservoirs (where more TSR should have occurred) and inferred delta(34)S values of H2S (from late-stage pyrite in the Carboniferous and Ordovician reservoirs) are as low as -24.9parts per thousand. Such low delta(34)5 values discount the decomposition of organic matter as a major source of H2S and CO2. Bacterial sulphate reduction of the light hydrocarbon gases in the reservoir, possibly coupled indirectly with the consumption of organic acids and anions is most likely. The result is the preferential oxidation of C-12-rich alkanes (due to the kinetic isotope effect) and decreasing concentration of organic acids and anions. Modern formation water stable isotope data reveal that it is possible that sulphate-reducing bacteria were introduced into the reservoir by an influx of meteoric water from the west by way of an inversion-related unconformity. This may account for the apparently stronger influence of bacterial sulphate reduction to the west of the Hetianhe Field, and the consequent greatest decrease of the delta(13)C-CO2 values and the greatest increase in delta(13)C values of the alkane gases. (C) 2002 Published by Elsevier Science Ltd.