ABSTRACT

Thermochemical sulfate reduction (TSR) is the reaction between anhydrite and petroleum fluids at elevated temperatures to produce H₂S and calcite. In this study of the dolomite-hosted hydrocarbon gas reservoirs in the Permo-Triassic Khuff Formation, Abu Dhabi, a geochemically well constrained rock-gas system, we demonstrate for the first time a clear influence of rock texture and mineralogy on the rate and extent of TSR reactions and thus on H₂S concentration in the gas phase. The controls on the rate on H₂S accumulation were:

1. TSR reaction kinetics. TSR became significant as temperature exceeded 140°C, a critical threshold temperature for chemical reaction between aqueous sulfate and aqueous methane.
2. Anhydrite dissolution rate. Once initiated, the subsequent reaction rate was controlled by the rate of supply of aqueous sulfate to the reaction site. Sulfate limitation is indicated by the lack of fractionation of sulfur isotopes between sulfate and sulfide (i.e., suggesting total reaction for each unit of sulfate that dissolves). Also, finely crystalline anhydrite began reacting at a lower temperature than coarse crystalline anhydrite (likely a result of relative surface area), suggesting that anhydrite dissolution was the rate-limiting step. Transport rates are unlikely to have been rate-limiting at the earliest stage of reaction because anhydrite was replaced in situ by calcite on the very edges of anhydrite nodules and crystals.
3. Transport rate. Later, as TSR proceeded, it became transport controlled, as calcite, growing on the surface of anhydrite crystals, began to isolate them from dissolved methane. TSR ceased once calcite had effectively armor-plated (totally isolated) the remaining anhydrite. Finely crystalline anhydrite underwent more extensive and more rapid TSR than coarser anhydrite crystals because these had a greater ratio of surface area to volume, allowing more and faster dissolution and requiring more calcite to isolate them from methane.

Localized loss of H₂S occurred in the reservoir by reaction with indigenous Fe-bearing clays. Consequently, reservoirs with a relatively high siliciclastic content have less H₂S than would be expected from the advanced state of anhydrite replacement by calcite. In order to predict TSR-related H₂S concentration in hydrocarbon gases it is thus important to understand the diagenetic and textural characteristics of the reservoir as well as the thermal and petroleum-emplacement history.