The role of thermochemical sulfate reduction in th

Authors: Kaikai Li Chunfang Cai Lianqi Jia Yang Gao Zaixing Jiang Tiankai Wang Lei Jiang

Publish Date: 2014/07/31

Volume: 8, Issue: 7, Pages: 4443-4456

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Abstract

Deeply buried Ordovician reservoirs within the central Tarim Basin China contain elevated concentrations of H2S This study uses petrography carbon and sulfur isotopes and fluid inclusion microthermometry data to determine whether these elevated concentrations formed as a result of thermochemical sulfate reduction TSR and evaluates the influence of TSR on porosity TSRderived H2S is likely to have been incorporated into pyrite and minor sphalerite as reflected by the similar δ34S values for H2S 150–234 ‰ and coarse crystalline pyrite 15–297 ‰ Sulfide precipitation caused an increase in acidity that allowed the dissolution of carbonates during TSR The occurrence of dissolution during deep burial is indicated by 1 the occurrence of dissolution pores within hightemperature 916–121 °C burial calcite cement 2 the coexistence of highporosity sediments with elemental sulfur and pyrite in areas away from unconformities that would have undergone only limited interaction with meteoric water 3 the presence of giant porous karstfilling calcites in association with coarse crystalline pyrite and 4 a slightly negative shift in the δ13C values of some postbitumen calcite Several factors probably limited the effect of TSR on the quality of reservoirs within the central Tarim Basin The reactive SO4 −2 involved in the TSR process was supplied by deep hydrothermal waters that contained dissolved sulfate rather than by the dissolution of anhydrite meaning that only a limited increase in TSRgenerated porosity occurred restricted to areas along faults The heterogeneity of metal ion concentrations within the reservoirs also constrained TSRrelated carbonate dissolution as after ferric ions within the reservoirs were consumed during advanced diagenesis TSRderived CO2 accumulated within the reservoirs causing a negative shift in presentday CO2 δ13C valuesThis work is financially supported by the National Science Foundation for Distinguished Young Scholars of China grant no 41202108 the 12th FiveYear National Petroleum Key Projects 41121690 and the Fundamental Research Funds for the Central Universities 2652013069 The authors thank an anonymous reviewer and the editor for giving us constructive suggestions which improved the quality of the paper

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