Abstract: Reactions Controlling H₂S Concentration in Deep Carbonate Reservoirs

Richard H. Worden, P. Craig Smalley

The economic viability of exploration and production from deep gas plays is often limited by the presence of hydrogen sulphide (H₂S). 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 degrees C, anhydrite has been partially replaced by calcite, 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 sulphur isotope data from the gases and minerals show that the dominant reaction is:

Equation (1)

Gas chemistry and isotope data also show that C[2+] gases reacted preferentially with anhydrite by reactions of the type:

Equation (2)

Sulphur was generated by this reaction and is locally present but was generally consumed by the reaction:

Equation (3)

The frequently quoted reaction between anhydrite and H₂S with CO₂ to produce calcite and sulphur, has been shown to be insignificant in the Khuff Formation by ^dgr¹³C and ^dgr³⁴S data.

Direct reaction between methane and anhydrite occurred in solution in residual pore waters initially dominated by dissolved carbonate derived from the marine dolomite matrix. The first formed replacive calcite contained carbon derived from the marine matrix (^dgr¹³C of about 0 ^pmil). Continuing reaction led to the progressive domination of carbonate derived from anhydrite reduction by methane (minimum ^dgr¹³C of about -31 ^pmil). 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 han 140°C, shows that equilibrium thermodynamic principles do not control gas souring reactions. Rather, the coexistence of anhydrite and methane in shallow reservoirs and the occurrence of a critical reaction temperature are kinetically controlled.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France