THE EFFECTS OF THERMOCHEMICAL SULFATE REDUCTION UPON FORMATION WATER SALINITY AND OXYGEN ISOTOPES IN CARBONATE GAS-RESERVOIRS

Thermochemical sulfate reduction (TSR) is a well known process that ca n lead to sour (H2S-rich) petroleum accumulations. Most studies of TSR have concentrated upon gas chemistry. In this study we have investiga ted palaeoformation water characteristics in a deep, anhydrite-bearing dolomite, sour-gas reservoir of Permian age in Abu Dhabi using fluid inclusion, stable isotope, petrographic, and gas chemical data. The da ta show that low salinity, isotopically-distinct water was generated w ithin the reservoir by reaction between anhydrite and methane. The amo unt of water added to the reservoir from TSR, indicated by reduced flu id inclusion salinity and water delta(18)O values, varied systematical ly with the extent of anhydrite reaction with methane. Water salinity and isotope data show that the original formation water was diluted by between four and five times by water from TSR. Thus, we have shown th at large volumes of very low salinity water were generated within the gas reservoirs during diagenesis following gas emplacement. The salini ty of formation water in evaporite lithologies is, therefore, not nece ssarily high. Modelling, based upon a typical Khuff gas reservoir rock volume, suggests that initial formation water volumes can only be inc reased by about three times as a result of TSR, The extreme local dilu tion shown by the water salinity and delta(18)O data must, therefore, reflect transiently imperfect mixing between TSR water and original fo rmation water. The creation of large volumes of water has important im plications for the mechanism and rate of thermochemical sulphate reduc tion and the interpretation of gas volumes using petrophysical logging tools.