THERMOCHEMICAL SULFATE REDUCTION - A REVIEW

The high concentrations of hydrogen sulfide found in many oil and gas fields is thought to arise from the oxidation of petroleum hydrocarbon s by sulfate - a reaction that reduces the value of the resource. This review, undertaken in order to better understand the geochemistry of TSR reaction in oil field sediments, covers the relevant information o n thermochemical sulfate reduction (TSR) to 1991. The theoretical and experimental aspects of TSR reactions (including sulfur and carbon iso tope studies) are reviewed and their significance to the geochemical s ystem discussed. The present review agrees with previous suggestions t hat biochemical reduction of sulfate dominates in sedimentary environm ents below 120-degrees-C, and supports the possibility that reactive s ulfur species will oxidize certain organic molecules at meaningful rat es in geochemically reasonable reaction periods at temperatures above 175-degrees-C. We conclude that under typical petroleum reservoir reac tion conditions, both elemental sulfur and polysulfides are capable of oxidizing some organic molecules under basic conditions. But that sul fate alone will not react unless lower oxidation state sulfur is prese nt. The possible interaction of low-valence-state sulfur with sulfate to form TSR active oxidants is examined, both H2S and SO4(2-) are requ ired for the formation of active polysulfide reductants (e.g. thiosulf ate or polythionates) in TSR systems. Such intermediates can serve to lower the overall activation energy of the oxidation of hydrocarbons b y sulfate via thermal generation of sulfur radicals that can function as TSR active oxidants in many oil field sediments. We suggest that so me proposed chemical mechanisms for TSR need to be experimentally veri fied and the results re-interpreted with respect to TSR reactions in g eologic systems.