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. TSR-derived H2S is likely to have been incorporated into pyrite and minor sphalerite, as reflected by the similar δ34S values for H2S (15.0–23.4 ‰) and coarse crystalline pyrite (15–29.7 ‰). 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 high-temperature (91.6–121 °C) burial calcite cement, (2) the coexistence of high-porosity 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 karst-filling 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 TSR-generated porosity occurred, restricted to areas along faults. The heterogeneity of metal ion concentrations within the reservoirs also constrained TSR-related carbonate dissolution, as after ferric ions within the reservoirs were consumed during advanced diagenesis, TSR-derived CO2 accumulated within the reservoirs, causing a negative shift in present-day CO2 δ13C values.
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