Characterizing Thermochemical Sulphate
Reduction: Towards Making Predictions of Sour Gas in Sedimentary Basins
Worden, Richard H.1 (1) University of Liverpool, Liverpool, United Kingdom
Thermochemical Sulphate
Reduction (TSR) involves the reaction between petroleum fluids and sulphate minerals in reservoirs and leads to the growth of
carbonate minerals and large quantities of H2S in the remaining petroleum. TSR
involves a large number of steps including mineral dissolution, the redox process, transport of
reactants and growth of mineral reaction products. It has not been possible to
predict the occurrence of TSR and so risk H2S occurrence in prospects. The
number of TSR case studies remains limited and the Permian-Triassic Khuff Formation in the Middle East remains one of the
better studied systems. A review of the data shows that oil-TSR occurs at
temperatures >110ºC while gas-TSR occurs at >140ºC showing that the
minimum temperature appears to sensitive to the type of petroleum in the
reservoir. Experimental determinations of TSR redox
rates have merely shown that the redox step is
unlikely to be rate-limiting since the determined half life of reactants is of
the order of 100 to 100,000 years whereas fields have contained co-existing sulphate minerals and petroleum for minerals of years
without reacting. Sulphur isotope data from sulphate and H2S surprisingly show no fractionation
suggesting that the delivery of sulphate (dissolution
of anhydrite) into the reactive milieu may be rate limiting. Furthermore, partly
reacted anhydrite nodules commonly develop an armour
plate of calcite that physically isolates remaining anhydrite from petroleum.
The rate limiting steps in TSR probably varies as reaction proceeds but,
despite vast experimental efforts, is unlikely to be the redox
step.