The association of dissolved sulfate and hydrocarbons is thermodynamically unstable in virtually all diagenetic environments. Hence, redox-reactions occur, whereby sulfate is reduced by hydrocarbons either bacterially (bacterial sulfate reduction = BSR) or inorganically (thermochemical sulphate reduction = TSR). Their geologically and economically significant products are similar. Based on empirical evidence, BSR and TSR occur in two mutually exclusive thermal regimes, i.e. low-temperature and high-temperature diagenetic environments, respectively. BSR is common in diagenetic settings from 0 up to about 60-80 degreesC. Above this temperature range, almost all sulfate-reducing microbes cease to metabolize. Those few types of hyperthermophilic microbes that can form H2S at higher temperatures appear to be very rare and do not normally occur and/or metabolize in geologic settings that are otherwise conducive to BSR. TSR appears to be common in geologic settings with temperatures of about 100-140 degreesC, but in some settings temperatures of 160-180 degreesC appear to be necessary. TSR does not have a sharply defined, generally valid minimum temperature because the onset and rate of TSR are governed by several factors that vary from place to place, i.e. the composition of the available organic reactants, kinetic inhibitors and/or catalysts, anhydrite dissolution rates, wettability, as well as migration and diffusion rates of the major reactants toward one another. BSR is geologically instantaneous in most geologic settings. Rates of TSR are much lower, but still geologically significant. TSR may form sour gas reservoirs and/or MVT deposits in several tens of thousands to a few million years in the temperature range of 100-140 degreesC. BSR and TSR may be exothermic or endothermic, depending mainly on the presence or absence of specific organic reactants. However, if the reactions are exothermic, the amount of heat Liberated is very small, and this heat usually dissipates quickly. Hence, heat anomalies found in association with TSR settings are normally not generated by TSR. The main organic reactants for BSR are organic acids and other products of aerobic or fermentative biodegradation. The main organic reactants for TSR are branched and n-alkanes, followed by cyclic and mono-aromatic species, in the gasoline range. Sulfate is derived almost invariably from the dissolution of gypsum and/or anhydrite, which may be primary or secondary deposits at or near the redox-reaction site(s). The products of BSR and TSR are similar, but their relative amounts vary widely and are determined by a number of locally variable factors, including availability of reactants, formation water chemistry, and wettability. The primary inorganic reaction products in both thermal regimes are H2S(HS-) and HCO3- (CO2). The presence of alkali earth metals often results in the formation of carbonates, particularly calcite and dolomite. Other carbonates, i.e. ankerite, siderite, witherite, strontianite, may form if the respective metal cations are available. Iron sulfides, galena, and sphalerite form as by-products of hydrogen sulfide generation, if the respective transition or base metals are present or transported to a BSR/TSR reaction site. Elemental sulfur may accumulate as a volumetrically significant net reaction product, usually when the system runs out of reactive hydrocarbons. Water may form as a by-product and might result in a local dilution of the formation waters at or near the reaction site. There are case studies of TSR however, where no dilution of the formation water has occurred, indicating that the amount of water released during TSR was negligible. Porosity may be generated during TSR, but most case studies show that TSR does not usually result in a significant increase in porosity. TSR is likely to take place in fairly narrow reaction zones, where the irreducible water saturation in the hydrocarbon-containing pores is low. In one well known case, the Nisku Formation of Alberta, Canada, this zone is about 10-20 m thick. However, where the irreducible water saturation is high, TSR may take place throughout the entire hydrocarbon-containing pore volume. Solid bitumen may form as a by-product of both BSR and TSR. The mere presence of any of the above reaction products and by-products does not permit a distinction between BSR and TSR However, a number of petrographic relationships and geochemical criteria can be used to discriminate these two processes. Specifically most solid products of BSR and TSR, although similar in gross composition can be distinguished from one another petrographically, i.e. most BSR (TSR) products form early (late) in the paragenesis and/or have distinctive crystal sizes, shapes, and/or reflectivity. However, there also are cases where BSR products formed relatively late diagenetically, e.g. in uplifted reservoirs after hydrocarbon migration. Gas chromatography, delta C-13-, delta S-34-analyses, and/or a combination thereof offer the best distinguishing geochemical criteria. (C) 2001 Elsevier Science B,V. All rights reserved.
