Hans G. Machel

Bacterial and thermochemical sulfate reduction in diagenetic settings — old and new insights

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 reductiona BSR) or inorganically (thermochemical sulphate reductiona 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 hightemperature diagenetic environments, respectively. BSR is common in diagenetic settings from 0 up to about 60‐808C. 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 100‐1408C, but in some settings temperatures of 160‐1808C 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 100‐1408C. 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 2 ) 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

Products and distinguishing criteria of bacterial and thermochemical sulfate reduction

Bacterial and thermochemical sulfate reduction apparently occur in two mutually exclusive thermal regimes, i.e., low-temperature diagenetic environments with0 < T < 60–80°C and high-temperature diagenetic environments with80–100 < T < 150–200°C, respectively. The major reaction products and by-products are identical in both thermal regimes and include altered and oxidized hydrocarbons (originally mainly crude oil, gas condensate, and/or methane), hydrogen sulfide, base and transition metal sulfides, elemental sulfur, and carbonates (mainly calcite and dolomite). The mere presence of the above reaction products and by-products does not discriminate between the low- and high-temperature diagenetic environments. However, petrographic, isotopic and compositional data of these products and by-products may permit identification of a bacterial versus a thermochemical origin. Regarding the inorganic phases, the carbon isotope ratios of the carbonates, sulfur isotope ratios of elemental sulfur and sulfides, and fluid inclusion data appear to yield the best discriminating geochemical criteria. Among the organic phases, reservoir bitumen and gas condensates display a number of useful isotopic and compositional (chromatographic) criteria. The most reliable approach for discriminating bacterial versus thermochemical sulfate reduction is to combine as many of these criteria as possible. These criteria can be used in exploration or deposits of hydrocarbons, sour gas, elemental sulfur, and certain metal sulfides.

Concepts and models of dolomitization: a critical reappraisal

Abstract Despite intensive research over more than 200 years, the origin of dolomite, the mineral and the rock, remains subject to considerable controversy. This is partly because some of the chemical and/or hydrological conditions of dolomite formation are poorly understood, and because petrographic and geochemical data commonly permit more than one genetic interpretation. This paper is a summary and critical appraisal of the state of the art in dolomite research, highlighting its major advances and controversies, especially over the last 20–25 years. The thermodynamic conditions of dolomite formation have been known quite well since the 1970s, and the latest experimental studies essentially confirm earlier results. The kinetics of dolomite formation are still relatively poorly understood, however. The role of sulphate as an inhibitor to dolomite formation has been overrated. Sulphate appears to be an inhibitor only in relatively low-sulphate aqueous solutions, and probably only indirectly. In sulphate-rich solutions it may actually promote dolomite formation. Mass-balance calculations show that large water/rock ratios are required for extensive dolomitization and the formation of massive dolostones. This constraint necessitates advection, which is why all models for the genesis of massive dolostones are essentially hydrological models. The exceptions are environments where carbonate muds or limestones can be dolomitized via diffusion of magnesium from seawater rather than by advection. Replacement of shallow-water limestones, the most common form of dolomitization, results in a series of distinctive textures that form in a sequential manner with progressive degrees of dolomitization, i.e. matrix-selective replacement, overdolomitization, formation of vugs and moulds, emplacement of up to 20 vol% calcium sulphate in the case of seawater dolomitization, formation of two dolomite populations, and — in the case of advanced burial — formation of saddle dolomite. In addition, dolomite dissolution, including karstification, is to be expected in cases of influx of formation waters that are dilute, acidic, or both. Many dolostones, especially at greater depths, have higher porosities than limestones, and this may be the result of several processes, i.e. mole-per-mole replacement, dissolution of unreplaced calcite as part of the dolomitization process, dissolution of dolomite due to acidification of the pore waters, fluid mixing (mischungskorrosion), and thermochemical sulphate reduction. There also are several processes that destroy porosity, most commonly dolomite and calcium sulphate cementation. These processes vary in importance from place to place. For this reason, generalizations about the porosity and permeability development of dolostones are difficult, and these parameters have to be investigated on a case-by-case basis. A wide range of geochemical methods may be used to characterize dolomites and dolostones, and to decipher their origin. The most widely used methods are the analysis and interpretation of stable isotopes (O, C), Sr isotopes, trace elements, and fluid inclusions. Under favourable circumstances some of these parameters can be used to determine the direction of fluid flow during dolomitization. The extent of recrystallization in dolomites and dolostones is much disputed, yet extremely important for geochemical interpretations. Dolomites that originally form very close to the surface and from evaporitic brines tend to recrystallize with time and during burial. Those dolomites that originally form at several hundred to a few thousand metres depth commonly show little or no evidence of recrystallization. Traditionally, dolomitization models in near-surface and shallow diagenetic settings are defined and/or based on water chemistry, but on hydrology in burial diagenetic settings. In this paper, however, the various dolomite models are placed into appropriate diagenetic settings. Penecontemporaneous dolomites form almost syndepositionally as a normal consequence of the geochemical conditions prevailing in the environment of deposition. There are many such settings, and most commonly they form only a few per cent of microcrystalline dolomite(s). Many, if not most, penecontemporaneous dolomites appear to have formed through the mediation of microbes. Virtually all volumetrically large, replacive dolostone bodies are post-depositional and formed during some degree of burial. The viability of the many models for dolomitization in such settings is variable. Massive dolomitization by freshwater-seawater mixing is a myth. Mixing zones tend to form caves without or, at best, with very small amounts of dolomite. The role of coastal mixing zones with respect to dolomitization may be that of a hydrological pump for seawater dolomitization. Reflux dolomitization, most commonly by mesohaline brines that originated from seawater evaporation, is capable of pervasively dolomitizing entire carbonate platforms. However, the extent of dolomitization varies strongly with the extent and duration of evaporation and flooding, and with the subsurface permeability distribution. Complete dolomitization of carbonate platforms appears possible only under favourable circumstances. Similarly, thermal convection in open half-cells (Kohout convection), most commonly by seawater or slightly modified seawater, can form massive dolostones under favourable circumstances, whereas thermal convection in closed cells cannot. Compaction flow cannot form massive dolostones, unless it is funnelled, which may be more common than generally recognized. Neither topography driven flow nor tectonically induced (‘squeegee-type’) flow is likely to form massive dolostones, except under unusual circumstances. Hydrothermal dolomitization may occur in a variety of subsurface diagenetic settings, but has been significantly overrated. It commonly forms massive dolostones that are localized around faults, but regional or basin-wide dolomitization is not hydrothermal. The regionally extensive dolostones of the Bahamas (Cenozoic), western Canada and Ireland (Palaeozoic), and Israel (Mesozoic) probably formed from seawater that was ‘pumped’ through these sequences by thermal convection, reflux, funnelled compaction, or a combination thereof. For such platform settings flushed with seawater, geochemical data and numerical modelling suggest that most dolomites form(ed) at temperatures around 50–80 °C commensurate with depths of 500 to a maximum of 2000 m. The resulting dolostones can be classified both as seawater dolomites and as burial dolomites. This ambiguity is a consequence of the historical evolution of dolomite research.

Chemistry and Environments of Dolomitization —A Reappraisal

Abstract Dolomitization of calcium carbonate can best be expressed by mass transfer reactions that allow for volume gain, preservation, or loss during the replacement process. Experimental data, as well as textures and porosities of natural dolomites, indicate that these reactions must include CO32− and/or HCO3− supplied by the solution to the reaction site. Since dolomite formation is thermodynamically favoured in solutions of (a) low Ca2+/Mg2+ ratios, (b) low Ca2+/CO32− (or Ca2+/HCO3−) ratios, and (c) high temperatures, the thermodynamic stability for the system calcite-dolomite-water is best represented in a diagram with these three parameters as axes. Kinetic considerations favour dolomitization under the same conditions, and additionally at low as well as at high salinities. If thermodynamic and kinetic considerations are combined, the following conditions and environments are considered chemically conducive to dolomitization: (1) environments of any salinity above thermodynamic and kinetic saturation with respect to dolomite (i.e. freshwater/seawater mixing zones, normal saline to hypersaline subtidal environments, hypersaline supratidal environments, schizohaline environments); (2) alkaline environments (i.e. those under the influence of bacterial reduction and/or fermentation processes, or with high input of alkaline continental groundwaters); and (3) many environments with temperatures greater than about 50°C (subsurface and hydrothermal environments). Whether or not massive, replacive dolostones are formed in these environments depends on a sufficient supply of magnesium, and thus on hydrologic parameters. Most massive dolostones, particularly those consisting of shallowing-upward cycles and capped by regional unconformities, have been interpreted to be formed according to either the freshwater/seawater mixing model or the sabkha with reflux model. However, close examination of natural mixing zones and exposed evaporitic environments reveals that the amounts of dolomite formed are small and texturally different from the massive, replacive dolostones commonly inferred to have been formed in these environments. Many shallowing-upward sequences are devoid of dolomite. It is therefore suggested that massive, replacive dolomitization during exposure is rare, if not impossible. Rather, only small quantities of dolomite (cement or replacement) are formed which may act as nuclei for later subsurface dolomitization. Alternatively, large-scale dolomitization may take place in shallow subtidal environments of moderate to strong hypersalinity. The integration of stratigraphic, petrographic, geochemical, and hydrological parameters suggests that the only environments capable of forming massive, replacive dolostones on a large scale are shallow, hypersaline subtidal environments and certain subsurface environments.

Saddle dolomite as a by-product of chemical compaction and thermochemical sulfate reduction

Petrographic, stable and radiogenic isotope, trace-element, and fluid-inclusion data from the Devonian Nisku reef trend in the subsurface of Alberta, Canada, suggest that saddle dolomite was a by-product of chemical compaction and thermochemical sulfate reduction in subsiding dolomitized rocks. Furthermore, thermochemical sulfate reduction may be representative of other processes that increase the amount of saddle dolomite via an increase in carbonate alkalinity. The data also confirm that saddle dolomite is most likely to form at elevated temperatures from hypersaline brines.

The influence of thermochemical sulphate reduction on hydrocarbon composition in Nisku reservoirs, Brazeau river area, Alberta, Canada

Abstract The Upper Devonian Nisku Formation reservoirs of the Brazeau river area of west-central Alberta, Canada, produce oil and sweet and sour gas condensate. Generally, oil pools are located updip in the study area and sour (6–31% H2S) gas condensate downdip. H2S in the study area is formed by thermochemical sulphate reduction (TSR). All liquid hydrocarbons probably have one source, with the Duvernay Formation being the most likely candidate. Assessing the relative maturity of the oils and condensates is difficult because of the wide variation in thermal maturity and the effects of TSR, but the ratio of pristane/n-heptadecane does appear to decrease with increasing maturity for this sample set. With increasing TSR, the following changes were noted: decrease in the saturate/aromatic hydrocarbon ratio; increase in the relative abundance of organo-sulphur compounds (e.g. benzothiophenes); δ34S values of liquid hydrocarbon samples approached the values for anhydrite of the Nisku Formation in the study area; and an increase in δ13C of the saturate fraction. H2S concentrations in hydrodynamically “isolated” pools provide a good estimate of the extent of TSR in these reservoirs. However, other pools have anomalously high H2S concentrations for their depth, suggesting that the H2S was generated at greater depths and migrated updip into these pools.

Hydrothermal dolomite—a product of poor definition and imagination

Abstract The latest dolomite bandwagon is the “hydrothermal dolomite model”. In its present form, this bandwagon is doomed or at least very much overstated for at least two reasons: (1) there are several definitions of hydrothermal, and hardly any author specifies which one s/he is using; (2) very few of the dolomites hitherto called hydrothermal have been demonstrated to be hydrothermal according to any definition, except the worst. As presently applied, the term “hydrothermal dolomite” is confusing and/or meaningless. We suggest to use Whites [Geol. Soc. Amer. Bull. 68 (1957) 1637] time-honored definition of “hydrothermal” as “aqueous solutions that are warm or hot relative to its surrounding environment”, with no genetic implications regarding the fluid source. Hence, a dolomite should be called hydrothermal only if it can be demonstrated to have formed at a higher than ambient temperature, regardless of fluid source or drive. Furthermore, this definition does not carry a lower or upper temperature limit. Even a dolomite formed at 40 °C could be hydrothermal. By extension, dolomites formed at temperatures lower than ambient are not hydrothermal, even if they formed at a rather high temperature. For example, groundwater may penetrate a rock sequence through a highly permeable pathway, such that it is heated to 150 °C at a depth where the surrounding rock has a temperature of 250 °C. We suggest to call dolomite formed from this water “hydrofrigid”. Dolomite formed in or near thermal equilibrium with the surrounding rocks may be called “geothermal”. Furthermore, not all saddle dolomite formation requires advection (fluid flow) to transport Mg. Saddle dolomite can be formed in at least three ways, i.e., from advection, local redistribution of older dolomite during stylolitization, and as a by-product of thermochemical sulfate reduction in a closed or semi-closed system. Only the first and the last of these three possibilities have a chance of being hydrothermal. Almost all dolomites and dolostones in the Western Canada Sedimentary Basin have recently been (re-)interpreted as hydrothermal. Applying the rationale outlined above reveals, however, that this basin contains very little hydrothermal dolomite. Rather, most dolomites in this basin, and almost all dolostones south of the Peace River Arch, are geothermal, and/or the proof of a hydrothermal origin has not been made. This has important implications beyond the various case studies at hand, as attempts to tie dolomitization to orogenic events become moot, at least in the southern part of the basin.

Luminescence Microscopy and Spectroscopy

The papers presented in this volume make it clear that luminescence microscopy and spectroscopy are being employed in an ever wider array of geological studies. The editors suggest several ways that luminescence studies can be employed or improved: (1) to assist in the integration of trace element, isotope, fluid inclusion and mineral studies using CL results to assure that the same zones and (or) mineral compositions are utilized; (2) more reliable tracing of zones whether microscopic or of regional extent; (3) better interpretation of diagenetic, mineralization and alteration events because of the control and discrimination of crosscutting relationships and subtle changes in chemistry that often become obvious using luminescence; (4) as a tool for direct detection of rare earth element deposits, Mississippi Valley type Pb-Zn ores and in some cases oil reservoirs; (5) introduction of standard materials and methods for calibrating spectrometers and possibly increasing the uniformity of subjective observations; (6) improvements in instrumentation to diminish thermal quenching effects at the same time gains are necessary in the level of activation of luminescence and in the quality of the microscopic image transmitted to the observer; (7) more efforts at experimental determination of the causes of luminescence and their interpretation relative to conditions that exist in natural systems.

Recrystallization versus neomorphism, and the concept of ‘significant recrystallization’ in dolomite research

Abstract The term ‘neomorphism’, originally introduced in 1965 as a substitute for the term ‘recrystallization’ in limestones, is now in use also for dolostones. However, only very few dolomite researches use the term neomorphism (even though it is well and strictly defined), many do not know what this term means, and using the term commonly leads to confusion among carbonate researchers, as well as between them and other geologists. Partially or largely for these reasons, most dolomite researchers use the term recrystallization rather than neomorphism. In addition, the term neomorphism does not encompass all the important characteristics of recrystallization known today. It is recommended, therefore to abandon the term neomorphism in dolomite research and apply the term recrystallization with an extended definition that includes those characteristics that commonly are involved in recrystallization and that can be measured in dolomites and dolostones: textural changes (size, shape), structural changes (ordering, strain), compositional changes (stoichiometry, isotopes, trace elements — including zoning, fluid inclusions), and changes in the paleomagnetic properties. For practical applications, the absence, presence and/or degree of recrystallization is often important for a genetic interpretation of dolostones. If changes via recrystallization in texture, structure, composition, and/or paleomagnetic properties results in data ranges that are larger than the original ones (which must be known for reference), a dolomite/ dolostone is said to be ‘significantly recrystallized’ and the reset properties are no longer representative of the fluid and environment of dolomitization but characterize the last event of recrystallization. However, not all measurable properties must be reset during recrystallization. In this case, inherited properties represent the event of dolomitization, whereas reset properties represent the fluids and process of recrystallization. In recrystallization resets only one of 10 properties (e.g., 87 Sr/ 86 Sr ratios) beyond the range of the pristine samples (within the analytical errors), this dolomite is significantly recrystallized with respect to Sr isotopes, yet insignificantly recrystallized with respect to the other nine properties. In either case, values of those properties that are identical to the pristine reference values can be used for genetic interpretations of dolomitization. The concept of ‘significant recrystallization’ is of great use in genetic interpretations of dolomites/dolostones. In particular, a reliable interpretation of the chemistry of the dolomitizing fluids no longer depends on an absolute recognition of recrystallization. Rather, it is sufficient to recognize that a dolomite is insignificantly recrystallized. Application of this principle also shows that many ancient dolomites and dolostones are insignificantly recrystallized. Finally, the concept of significant recrystallization is independent of mineralogy. Hence, it can and should be applied also to limestones.

Some aspects of diagenetic sulphate-hydrocarbon redox reactions

Summary Sulphate-hydrocarbon redox-reactions occur at two specific diagenetic temperature/thermal maturity levels: less than about 75–85°C (0.2–0.3% R0), and more than 100–140°C (> 1.5% R0), respectively. In low-temperature/maturity environments these redox reactions take place only with the mediation of bacteria. In high-temperature/maturity environments these reactions take place thermochemically, and certain catalysts must interact in order to overcome the high activation energies and to sustain the reactions at geologically significant rates. The reaction products and by-products may be identical for both temperature/maturity levels: altered and oxidized hydrocarbons (including bitumen), hydrogen sulphide, metal sulphides (including Mississippi Valley Type deposits), elemental sulphur, carbonates (mainly calcite and dolomite), and other minerals. An important by-product of these redox reactions may be porosity resulting from the dissolution of solid sulphates and/or the carbonate host rock. The net reaction is exothermic, and the released heat may generate a geothermal hot-spot in some cases.