High concentrations of H2S in petroleum gas and oil fields are attributed to Thermochemical Sulphate Reduction (TSR) reactions. H2S is toxic, Corrosive towards production steels, commonly associated with high carbon dioxide concentrations and consequently low hydrocarbon concentrations. High H2S concentrations may affect the economic viability of a gas or oil field. In this thesis, the reaction kinetics and mechanism of TSR are determined in laboratory simulations at formation water pH. In addition, high-resolution electron-optical techniques have been used to characterise TSR textures in natural, anhydrite-bearing rock samples. TSR experiments have been performed in the sodium sulphate - acetic acid - sodium acetate - elemental sulphur system, at temperatures between 100 and 350"C. Sodium sulphate was used as an appropriate experimental analogue for anhydrite. Reactions have also been performed using sodium bisulphate, dextrose, oxalic acid and hydrogen. Fluid-sampling hydrothermal pressure vessels were used, with which pressure and temperature could be controlled independently up to 50 MPa (500 bars; 7500 psi) and 350'C. The reactants were held within a gold reaction cell, with titanium closure and exit tube which leads to an external titanium sampling valve. Fluid samples were taken periodically to monitor reaction progress without quenching the reaction vessel to room conditions. The rate law for the temperature dependence of the TSR reaction rate (min") between sodium sulphate, acetic acid and sodium acetate, in the presence of elemental sulphur is: Log k= -7.42 (103 /T) + 8.46 Kinetic data suggest that TSR occurs rapidly on a geological time-scale at relatively low temperatures( 150'C). Sulphate half-lives are 15 days at 300"C, and 1650 years at 150'C. The activation energy of the reaction is 142 W/mol. TSR is a first order reaction with respect to sodium sulphate, acetic acid and hydrogen ion activity. Fluid pH increases by 1/2-1 unit during these reactions. Minimal sulphate reduction is observed when catalytic elemental sulphur concentrations (2-3% of ∑S) are used in the reaction. TSR occurs readily in experiments with an elemental sulphur concentration in excess of 22% of ∑S. Periodic sampling of reaction fluids has enabled time-variant isotopic determinations to be produced for the first time. Elemental sulphur undergoes hydrolysis (disproportionation) to generate hydrogen sulphide and sulphate. At in situ pH (5.23-6.05), bisulphate is the dominant oxidised sulphur species. However, a low equilibrium concentration of fully protonated sulphate is present. Neutrally charged H2SO4 undergoes reaction with hydrogen sulphide to form a thiosulphate reactive intermediate and water. This reaction is reversible and decomposition of thiosulphate to sulphate and H2S allows for sulphur isotope exchange between sulphate and hydrogen sulphide and exchange of oxygen between sulphate and water. Sulphur fractionation factors of 1.019-1.020 have been determined for the reduction of aqueous sulphate by acetic acid and sodium acetate, in the presence of elemental sulphur. Equilibrium oxygen isotopic exchange is attained in approximately I hour at 300'C. In acid conditions, thiosulphate decomposes to form nascent (monatornic) sulphur and sulphite. This reaction maintains a low concentration of nascent sulphur, which is the species that probably undergoes further reactions with organic species to form H2S and C02- Petrographically altered anhydrite from the Permo-Triassic Khuff Formation (offshore Abu Dhabi) has been investigated using optical microscopy, SEM and TEM. A framework of reaction textures that are indicative of varying degrees of TSR is presented. TEM analysis of apparently unreacted samples has identified the presence of small calcite crystals at the interfaces between nodular anhydrite and matrix dolomite. This is interpreted to represent fine-scale replacement of anhydrite by calcite and raises the possibility of using core material as a petrographic indicator of TSR. No orientation relationship was found between TSR calcite and any other reservoir phase.