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 constrai ns the reactions responsible for the origin of H2S-rich gas in a class ic sour gas province: the Permian Khuff Formation of Abu Dhabi. In res ervoirs hotter than 140 degrees C, anhydrite has been partially replac ed by calcite, and hydrocarbon gases have been partially or fully repl aced by H2S. This shows that anhydrite and hydrocarbons have reacted t ogether to produce calcite and H2S. Carbon and elemental sulphur isoto pe data from the gases and minerals show that the dominant reaction is : CaSO4 + CH4 --> CaCO3 + H2S + H2O Gas chemistry and isotope data als o show that C-2+ gases reacted preferentially with anhydrite by reacti ons of the type: 2CaSO(4) + C2H6 --> 2CaCO(3) + H2S + S + 2H(2)O Sulph ur was generated by this reaction and is locally present but was also consumed by the reaction: 4S + CH4 + 2H(2)O --> CO2 + 4H(2)S The frequ ently quoted and experimentally-observed reaction between anhydrite an d H2S with CO2 to produce calcite and sulphur: CaSO4 + 3H(2)S + CO2 -- > CaCO3 + 4S + 3H(2)O has been shown to be insignificant in the Rhuff Formation by gas chemistry, calcite delta(13)C and sulphur delta(34)S data. Direct reaction between methane and anhydrite occurred in soluti on, in residual pore waters which were initially dominated by dissolve d carbonate derived from the marine dolomite matrix. The first-formed replacive clacite thus contains carbon derived principally from the ma rine dolomite matrix (delta(13)C of about 0 to +4 parts per thousand). Continuing reaction led to the progressive domination of the water by TSR-derived carbonate (minimum delta(13)C of about -31 parts per thou sand). Thermodynamic modeling using gas fugacity data was used to asse ss the controls on gas souring. To maintain equilibrium, anhydrite and methane should react together to produce calcite and H2S at all tempe ratures greater than 25 degrees C. The coexistence of unreacted anhydr ite and methane at shallow depths, in reservoirs than 140 degrees C, s hows that thermodynamics alone do not control gas souring reactions. R ather, the coexistence of anhydrite and methane in shallow reservoirs and their reaction to produce H2S are kinetically controlled.