Methane-dominated thermochemical sulphate reduction in the Triassic Feixianguan formation East Sichuan Basin, China:: towards prediction of fatal H2S concentrations

New sour pools have recently found in the Lower Triassic Feixianguan Fm carbonate reservoirs in the East Sichuan Basin in China with H2S up to 17.4% by volume. A recent blowout from a well drilled into this formation killed hundreds of people as a result of the percentage concentrations of H2S. In order to assess the origin of fatal H2S as well as the cause of petroleum alteration. H2S concentrations and the isotopes, delta(34)S and delta(13)C have been collected and measured in gas samples from reservoirs. Anhydrite, pyrite and elemental sulphur delta(34)S values have een measured for comparison. The high concentrations of H,S gas are found to occur at depths > 3000 m (temperature now at similar to 100 degreesC in evaporated platform facies oolitic dolomite or limestone that contains anhydrite nodule occurrence within the reservoirs. Where H2S concentrations are greater than 10% its delta(34)S values lie between + 12.0 and + 13.2parts per thousand CDT. This is within the range of anhydrite delta(34)S values found within the Feixianguan Fm (+ 11.0 to + 21.7parts per thousand; average 15.5 +/- 3.59parts per thousand CDT). Thus H2S must have been generated by thermochemical sulphate reduction (TSR) locally within the reservoirs. Burial history analysis and fluid inclusion data reveal that the temperature at which TSR occurred was greater than about 130-140 degreesC, suggesting that the present depth-temperature minimum is an artifact of post-TSR uplift. Both methane and ethane were actively involved in TSR since the petroleum became almost totally dry (no alkanes except methane) and methane delta(13)C values become significantly heavier as TSR proceeded. Methane delta(13)C difference thus reflects the extent of TSR. While it is tempting to use a present-day depth control (> 3000 m) to predict the distribution of H2S in the Feixianguan Fm, this is an invalid approach since TSR occurred when the formation was buried some 1000-2000 m deeper than it is at present. The likelihood of differential uplift across the basin means that it is important to develop a basinal understanding of the thermal history of the Fexianguan Fm so that it is possible to determine which parts of the basin have been hotter than 130-140 degreesC. (C) 2004 Elsevier Ltd. All rights reserved.