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.

Concepts and models of dolomitization: a critical reappraisal

A mathematically simple kinetic model simulates quartz cementation and the resulting porosity loss in quartzose sandstones as a function of temperature history. Dissolved silica is considered to be sourced from quartz dissolution at stylolites or individual quartz grain contacts containing clay or mica, and diffuses short distances to sites of precipitation on clean quartz surfaces. The modeled sandstone volume is located between stylolites, and no quartz dissolution or grain interpenetration takes place within this volume. After quartz cementation starts, compactional porosity loss is typically minor, and porosity loss within the modeled sandstone volume is therefore considered to be equal to the volume of precipitated quartz cement. The quartz cementation process is mod led as a precipitation rate-controlled reaction where quartz precipitation rate per unit time and surface area can be expressed by an empirically determined logarithmic function of temperature. When the sandstone's temperature history is known, precipitation rate per unit time and surface area can be expressed as a function of time, and the amount of quartz cement precipitated within a certain time interval can be calculated by multiplying the precipitation rate function with the surface area available for quartz precipitation and integrating with respect to time. Because quartz surface area will change as quartz cement precipitation proceeds, the calculations are performed for short time steps, and quartz surface area is adjusted after each time step. The total amount of quartz cement p ecipitated during a sandstone's burial history and the corresponding porosity loss are found by taking the sum of the increments of quartz cement precipitated during each time step. The effect of variation in parameters such as grain size, detrital quartz content, abundance of clay or other grain coatings, prequartz cementation porosities, and temperature history is easily simulated with the presented algorithm. This flexibility is illustrated by presenting calculated histories of quartz cementation and porosity loss for sandstones with a range of grain sizes, framework grain compositions, degree of clay coat development, prequartz cementation porosities, and temperature histories.

Kinetic Modeling of Quartz Cementation and Porosity Loss in Deeply Buried Sandstone Reservoirs

Descriptions of mineralogy and textural relationships in sandstones and limestones have been used to establish a sequence of diagenetic events (epigenesis), involving mineral dissolution and precipitation, which have been interpreted to have occurred during the burial history. Published epigenetic sequences commonly imply a geochemically open system with very significant changes in the bulk chemical composition of the sediments during burial. Near-surface diagenetic reactions may be open, involving significant changes in the sediment composition and formation of secondary porosity caused by high pore-water flow rates of meteoric water or reactions with sea water near the sea floor. Calculations show that the bulk chemical composition of the sediments below the reach of high pore-water flow rates of meteoric water or hydrothermal convection should remain nearly constant during progressive burial because of limited pore-water flow. Mass transport between shales and sandstones is also limited because the pore water is, in most cases, buffered by the same minerals so that the concentration gradients are low. Recent studies show that silica released from clay-mineral reactions in mudstones has been precipitated locally as small quartz crystals and not exported to adjacent sandstones. If the geochemical constraints for mass transfer during burial diagenetic reactions are accepted, the chemical reactions involved in diagenesis can be written as balanced equations. This offers the possibility to make predictions about reservoir quality based on assumptions about primary sediment composition related to facies and provenance. Large-scale changes in the bulk composition of sandstones and mudstones during burial diagenesis have been suggested, but because such changes cannot be explained chemically and physically, no predictions can be made. Burial diagenetic processes are, in most cases, not episodic but occur as slow adjustments to increased stress and temperature, driving the sediments toward increased mechanical and thermodynamic stability. As a result, the porosity of a single lithology must decrease during progressive burial, but each lithology has a different porosity curve. This article discusses quantitative calculations and estimates that show clearly that burial diagenesis must represent geochemically nearly closed systems where mineral dissolution and precipitation must be balanced. This provides a theoretical basis for the modeling and prediction of reservoir quality.

Open or closed geochemical systems during diagenesis in sedimentary basins: Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs

Vertical convection in an aquifer column under a gas cap of CO2

Geoscientists have always been interested in temperature values, as the crucial role of temperature in various geological, geochemical, and geophysical phenomena was recognized relatively early However, this interest was long limited to aspects such as temperature distribution in the earth’s crust and mantle, melting temperatures of different rocks, equilibrium temperatures of various metamorphic mineral assemblages, and temperature dependency of some geophysical parameters Almost all of these are related to thermal conditions in the deeper parts of the earths crust Low temperature fields at shallow depth have been of less concern It is interesting to recognize that all these studies sought to define either the present distribution of temperature or the maximum temperature reached at some time in the geological past In other words, temperature distribution as a steady state case was the subject of interest

A. Mo

Papers

Modelling of thermal convection in sedimentary basins and its relevance to diagenetic reactions