Thermochemical Sulfate Reduction in the Tazhong District,Tarim Basin,Northeast China: Evidence from Formation Water and Natural Gas Geochemistry

Systematic analyses of the formation water and natural gas geochemistry in the Central Uplift of the Tarim Basin (CUTB) show that gas invasion at the late stage is accompanied by an increase of the contents of H₂S and CO₂ in natural gas, by the forming of the high total dissolved solids formation water, by an increase of the content of HCO^?₃, relative to Cl^?, by an increase of the 2nd family ions (Ca²⁺, Mg²⁺, Sr²⁺ and Ba²⁺) and by a decrease of the content of SO^2?₄, relative to Cl^?. The above phenomena can be explained only by way of thermochemical sulfate reduction (TSR). TSR often occurs in the transition zone of oil and water and is often described in the following reaction formula: σCH+CaSO₄+H‐₂O→H₂S+CO₂+CaCO₃. (1) Dissolved SO^2?₄ in the formation water is consumed in the above reaction, when H₂S and CO₂ are generated, resulting in a decrease of SO^2?₄ in the formation water and an increase of both H₂S and CO₂ in the natural gas. If formation water exists, the generated CO₂ will go on reacting with the carbonate to form bicarbonate, which can be dissolved in the formation water, thus resulting in the enrichment of Ca²⁺ and HCO^?₃. The above reaction can be described by the following equation: CO₂+H₂O+CaCO₃→Ca²⁺+2HCO^?₃. The stratigraphic temperatures of the Cambrian and lower Ordovician in CUTB exceeded 120°C, which is the minimum for TSR to occur. At the same time, dolomitization, which might be a direct result of TSR, has been found in both the Cambrian and the lower Ordovician. The above evidence indicates that TSR is in an active reaction, providing a novel way to reevaluate the exploration potentials of natural gas in this district.