Formation water chemistry, sulfate sulfur isotopes, and associated H 2 S contents and sulfur isotopes were measured from the Ordovician in Tazhong area, Tarim Basin. The aim is to elucidate the effects of geochemical composition of formation water on thermochemical sulfate reduction (TSR) and potential usage of SO 4 /Cl ratios as a new proxy for TSR extents in areas, where H 2 S and thiaadamantanes (TAs) data are not available. The formation water has SO 4 /Cl ratios from 0.0002 to 0.016, significantly lower than 0.04 to 0.05 from 3 to 7 times evapoconcentrated seawater. Thus, the low values are explained to result from TSR. Furthermore, the SO 4 /Cl ratios show negative correlation relationships to TAs and H 2 S concentrations, indicating that TSR occurred in a relatively closed system and SO 4 /Cl ratio can be used to indicate TSR extents in this area. Extensive TSR in the Cambrian in the Tazhong area, represented by low SO 4 /Cl ratios and high H 2 S and TAs concentrations, is accompanied by formation water with high TDS and Mg concentrations, indicating the effects of water chemistry on TSR under a realistic geological background. In contrast, the low TSR extent in the Ordovician may have resulted from limited TSR reaction duration and total contribution of aqueous SO 4 2− .