H2S-producing reactions in deep carbonate gas reservoirs: Khuff Formation, Abu Dhabi

The economic viability of gas production from deep reservoirs is often limited by the presence of hydrogen sulphide (H2S) thought to be the result of thermochemical sulphate reduction (TSR). This study constrains the reactions responsible for the origin of H2S-rich gas in a classic sour gas province: the Permian Khuff Formation of Abu Dhabi. In reservoirs hotter than 140°C, anhydrite has been partially replaced by calcite, and hydrocarbon gases have been partially or fully replaced by H2S. This shows that anhydrite and hydrocarbons have reacted together to produce calcite and H2S. Carbon and elemental sulphur isotope data from the gases and minerals show that the dominant reaction is: CaSO4 + CH4 → CaCO3 + H2S + H2O Gas chemistry and isotope data also show that C2+ gases reacted preferentially with anhydrite by reactions of the type: 2CaSO4 + C2H6 → 2CaCO3 + H2S + S + 2H2O Sulphur was generated by this reaction and is locally present but was also consumed by the reaction: 4S + CH4 +2H2O → CO2 4H2S The frequently quoted and experimentally-observed reaction between anhydrite and H2S with CO2 to produce calcite and sulphur: CaSO4 + 3H2S + CO2 → CaCO3 + 4S + 3H2O has been shown to be insignificant in the Khuff Formation by gas chemistry, calcite δ13C and sulphur δ34S data. reaction between methane and anhydrite occurred in solution, in residual pore waters which were initially dominated by dissolved carbonate derived from the marine dolomite matrix. The first-formed replacive calcite thus contains carbon derived principally from the marine dolomite matrix (δ13C of about 0 to +4‰). Continuing reaction led to the progressive domination of the water by TSR-derived carbonate (minimum δ13C of about −31‰). Thermodynamic modelling using gas fugacity data was used to assess the controls on gas souring. To maintain equilibrium, anhydrite and methane should react together to produce calcite and H2S at all temperatures greater than 25°C. The coexistence of unreacted anhydrite and methane at shallow depths, in reservoirs cooler than 140°C, shows that thermodynamics alone do not control gas souring reactions. Rather, the coexistence of anhydrite and methane in shallow reservoirs and their reaction to produce H2S are kinetically controlled.