Thermochemical sulphate reduction in Cambro-Ordovician carbonates in Central Tarim

H2S and CO2 are found in elevated concentrations in Palaeozoic reservoirs i n the Tarim Basin in China. We have carried out analyses on gas, petroleum, mineral cement and bulk rock compositions and isotope ratios together with petrography and fluid inclusion to assess the origin of the H2S. A deep cr ustal (e.g. volcanic) origin of the H2S and CO2 is unlikely since the inert gases, N-2 and He, have isotope ratios totally uncharacteristic of this so urce. Organic sources are also unlikely since the source rock has low a sul phur content and the sulphur isotope ratio of the petroleum correlates posi tively with the sulphur content, the opposite of what would be anticipated from petroleum-derived H2S. Bacterial sulphate reduction is unlikely becaus e temperatures are too high for bacteria to have survived. Thermochemical s ulphate reduction of petroleum fluids by anhydrite in Lower Ordovician and Cambrian carbonate reservoirs is the most likely source of both the H2S and the CO2 causing isotopically characteristic Pyrite, CO2 gas and calcite ce ment. H2S, and possibly CO2, migrated into Silurian sandstone reservoirs by cross formational flow. The H2S, with the same sulphur isotope ratio as Or dovician anhydrite, was partially lost from the fluid phase by extensive gr owth of late diagenetic pyrite. Similarly the CO2 was partially lost from t he fluid phase by precipitation of late diagenetic calcite. The H2S that re sulted from TSR underwent reaction with the remaining petroleum resulting i n locally elevated organic sulphur concentrations in the petroleum and the progressive adoption of the Ordovician anhydrite sulphur isotope ratio. (C) 2001 Elsevier Science Ltd. All rights reserved.