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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)

Abstract


Journal of Sedimentary Research, Section A: Sedimentary Petrology and Processes
Vol. 70 (2000), No. 5. (September), Pages 1210-1221

The Influence of Rock Fabric and Mineralogy on Thermochemical Sulfate Reduction: Khuff Formation, Abu Dhabi

R.H. Worden (1) (*), P.C. Smalley (2), M.M. Cross (3)

ABSTRACT

Thermochemical sulfate reduction (TSR) is the reaction between Previous HitanhydriteNext Hit 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 sim.gif (57 bytes) 140°C, a critical threshold temperature for chemical reaction between aqueous sulfate and aqueous methane.
  2. Previous HitAnhydriteNext Hit 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 Previous HitanhydriteNext Hit began reacting at a lower temperature than coarse crystalline Previous HitanhydriteNext Hit (likely a result of relative surface area), suggesting that Previous HitanhydriteNext Hit dissolution was the rate-limiting step. Transport rates are unlikely to have been rate-limiting at the earliest stage of reaction because Previous HitanhydriteNext Hit was replaced in situ by calcite on the very edges of Previous HitanhydriteNext Hit nodules and crystals.
  3. Transport rate. Later, as TSR proceeded, it became transport controlled, as calcite, growing on the surface of Previous HitanhydriteNext Hit crystals, began to isolate them from dissolved methane. TSR ceased once calcite had effectively armor-plated (totally isolated) the remaining Previous HitanhydriteNext Hit. Finely crystalline Previous HitanhydriteNext Hit underwent more extensive and more rapid TSR than coarser Previous HitanhydriteNext Hit 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 Previous HitanhydriteTop 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.


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