Effects of groundwater flow on mineral diagenesis, with ... - Springer Link

Effects of groundwater flow on mineral diagenesis, with emphasis on carbonate aquifers Hans G. Machel

Abstract This article provides a critical synopsis of the effects of groundwater flow on mineral diagenesis. Emphasis is placed on those aspects and processes that change porosity and permeability in carbonate aquifers, because they are of particular importance to human societies as sources of supplies of water for human consumption (drinking, irrigation) and of crude oil and natural gas. Diagenetic settings in carbonates as well as clastics are generally ill defined. This paper proposes a new comprehensive classification of diagenetic settings into near-surface, shallow-, intermediate-, and deep-burial diagenetic settings; hydrocarbon-contaminated plumes; and fractures. These settings are defined on the basis of mineralogy, petroleum, hydrogeochemistry, and hydrogeology. This classification is applicable to all sedimentary basins. Diagenesis is governed by various intrinsic and extrinsic factors that include thermodynamic and kinetic constraints, as well as microstructural factors that may override the others. These factors govern diagenetic processes, such as dissolution, compaction, recrystallization, replacement, and sulfate–hydrocarbon redox-reactions. Processes such as cementation, dissolution, and dolomitization require significant flow of groundwater driven by an externally imposed hydraulic gradient. Other processes, such as stylolitization and thermochemical sulfate reduction, commonly take place without significant groundwater flow in hydrologically nearly or completely stagnant systems that are geochemically “closed.” Two major effects of groundwater flow on mineral diagenesis are enhancement and reduction of porosity and permeability, although groundwater flow can also leave these rock properties essentially unchanged. In extreme cases, an aquifer or hydrocarbon reservoir

Received, April 1998 Revised, July 1998 Accepted, September 1998 Hans G. Machel Department of Earth and Atmospheric Sciences University of Alberta, Edmonton, AB T6G 2E3 Canada Fax: c1-780-492-2030 e-mail: hans.machel6ualberta.ca Hydrogeology Journal (1999) 7 : 94–107

rock can have highly enhanced porosity and permeability due to extensive mineral dissolution, or it can be plugged up due to extensive mineral precipitation. Résumé Cet article donne un résumé critique des effets de l’écoulement des eaux souterraines sur la diagenèse minérale. L’accent est mis sur les aspects et les processus qui modifient la porosité et la perméabilité des aquifères carbonatés, parce qu’ils présentent une importance particulière pour les collectivités par leurs ressources en eau (eau potable, irrigation) et par le pétrole et le gaz naturel qu’ils renferment. Les conditions de diagenèse dans les carbonates comme dans les roches détritiques sont en général mal définies. Ce papier propose une nouvelle classification d’ensemble des conditions de diagenèse dans des situations proches de la surface, peu profondes, intermédiaires et profondément enfouies, dans les panaches de pollution par les hydrocarbures et dans des fractures. Ces conditions sont définies en fonction de la minéralogie, du pétrole, de l’hydrogéochimie et de l’hydrogéologie. Cette classification est applicable à tous les bassins sédimentaires. La diagenèse est commandée par divers facteurs intrinsèques et extrinsèques, parmi lesquels les contraintes thermodynamiques et cinétiques des facteurs microstructuraux peuvent s’imposer aux autres. Ces facteurs commandent les processus de diagenèse comme la dissolution, la compaction, la recristallisation, les échanges et les réactions redox concernant les sulfates et les hydrocarbures. Les processus tels que la cimentation, la dissolution et la dolomitisation nécessitent un écoulement important d’eau souterraine commandé par un gradient hydraulique fixé par des conditions externes. D’autres processus, tels que la stylolitisation et la réduction thermochimique des sulfates, se produisent habituellement sans écoulement souterrain significatif dans des systèmes presque ou totalement stagnants qui sont géochimiquement «fermés». Les deux effets dominants des écoulements souterrains sur la diagenèse minérale sont l’accroissement et la diminution de la porosité et de la perméabilité, même si l’écoulement souterrain peut par ailleurs conserver pour l’essentiel intactes les propriétés de la roche. Dans des cas extrêmes, une roche aquifère ou un Q Springer-Verlag

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réservoir pétrolier peut subir une forte augmentation de porosité et de perméabilité du fait d’une dissolution poussée des minéraux, ou bien être cimenté par une importante précipitation de minéraux. Resumen Este artículo proporciona una sinopsis crítica de los efectos del flujo de agua subterránea sobre la diagénesis mineral. Se enfatizan aquellos aspectos y procesos que inducen a cambios en la porosidad y permeabilidad de los acuíferos carbonatados, ya que éstos son de gran importancia como fuentes de abastecimiento de agua (para consumo humano o irrigación), petróleo o gas. Los ambientes diagenéticos en carbonatos o rocas clásticas suelen estar mal definidos. En este artículo se propone una nueva clasificación exhaustiva de los ambientes diagenéticos, que se subdividen en cuasisuperficiales, poco-, mediana- o profundamente enterrados, contaminados por hidrocarburos y fracturas. La división se hace a partir de los datos sobre mineralogía, petróleo, hidrogeoquímica e hidrogeología. Esta clasificación es aplicable a cualquier cuenca sedimentaria. La diagénesis está gobernada por factores intrínsecos y extrínsecos, incluyendo los cinéticos, termodinámicos y estructurales (que pueden ser los más limitantes). Estos factores gobiernan algunos procesos diagenéticos: disolución, compactación, recristalización, intercambio y reacciones redox sulfato-hidrocarburos. Otros procesos como cementación, disolución y dolomitización requieren un flujo importante de agua subterránea. En cambio, procesos como la estilolitización y la reducción termoquímica de los sulfatos suelen tener lugar con flujos hidrogeológicos pequen˜os, en zonas de estancamiento que están “cerradas” geoquímicamente. Los dos efectos más importantes del agua subterránea sobre la diagénesis mineral son el aumento y la disminución de porosidad y permeabilidad, aunque en algunos casos aun en presencia de agua las propiedades de la roca permanecen prácticamente inalteradas. En casos extremos, un acuífero o una roca petrolífera pueden llegar a tener una altísima porosidad y permeabilidad debido a la disolución de minerales, o bien puede colmatarse por precipitación del mineral. Key words groundwater flow 7 mineral diagenesis 7 carbonate rocks 7 hydraulic properties

Introduction Mineral diagenesis in carbonate and clastic rocks is governed by many factors, most notably mineralogy, temperature, pressure, water composition (including saturation states, redox-potential, and partial pressures of dissolved gases), flow rates, dissolution and precipitation kinetics, availability of nucleation sites, porosity, permeability, and (in hydrocarbon-bearing systems) wettability. These factors can be grouped into three major, overriding factors: mineralogy/petrology, hydroHydrogeology Journal (1999) 7 : 94–107

geochemistry, and hydrogeology. Mineralogy/petrology, i.e., the original composition and framework of the rocks, determines what minerals may form or dissolve in a given rock during diagenesis. For example, calcium carbonate can only dissolve where calcium carbonate is present, and quartz may or may not precipitate when the pore fluids are supersaturated with respect to quartz, depending on the surface area and accessibility of suitable nucleation sites in the rock. Hydrogeochemistry is a major governing factor because of two aspects. Firstly, diagenesis cannot take place without the presence of an aqueous fluid, because solid-state diffusion rates are negligibly small, even over billions of years, at the p/T conditions of the diagenetic realm. Secondly, thermodynamics (via saturation states) govern whether minerals can dissolve or precipitate. The role of hydrogeology, i.e., the movement of groundwater, is known from petrographic investigations of numerous sedimentary rocks, which reveal that significant amounts of porosity (commonly about 20–40% by volume) have been gained or lost through mineral dissolution or precipitation. Considering that the aqueous solubilities of most rock-forming minerals are quite small (ion activity products of ~~10 –6 moles/ L), large volumes of water must have flowed through the rocks in order to facilitate the observed changes in mineral volumes. These considerations further indicate that mineralogy/petrology, hydrogeochemistry, and hydrogeology are interdependent. In other words, these factors almost invariably interact to cause the diagenetic changes that affect the rocks (e.g., Bjørlykke 1996). These facts provide the basis for this article and its major objective, which is to provide a synopsis of the effects of groundwater flow on mineral diagenesis. The approach taken toward presenting these effects (e.g., generation of a certain type of porosity or a certain type of cement) of groundwater flow is via the causative factors and processes (e.g., undersaturation because of one factor, supersaturation because of another). This approach shows how mineralogy/petrology, hydrogeochemistry, and hydrogeology interact, and to which degree groundwater flow is involved in shaping the effects on the rocks. Compiling the material for the major objective of this paper, it became apparent that diagenetic burial settings are often ill defined and/or incompatible. Hence, a second objective arose, i.e., to provide a generally applicable classification of diagenetic environments. Ideally, this article should cover the diagenetic effects of groundwater flow in carbonate and clastic aquifers and aquitards. Due to space limitations, however, the scope of this article is limited to (1) aquifers and (2) carbonates. Emphasis is placed on aquifers because they are of particular importance to human societies in providing supplies of water for human consumption (drinking, irrigation), and supplies of crude oil and natural gas. In the case of water for Q Springer-Verlag

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human consumption, mineral diagenesis in aquifers causes mainly increases or decreases in groundwater quantity (flux), and, to a minor degree, in water quality. In the case of hydrocarbons, diagenesis may determine or alter the migration pathways and rates, as well as trap capacities. In extreme cases, an aquifer or hydrocarbon reservoir rock can have highly enhanced porosity and permeability due to extensive mineral dissolution, or it can be plugged up due to extensive mineral precipitation. Most of these aspects are addressed below, except for changes in water quality (discussed by Stuyfzand 1999; this volume) and hydrocarbon migration (see England 1994, and references cited therein). Furthermore, this article is focussed on carbonates, because this writer’s expertise is mainly in carbonate diagenesis and carbonate hydrocarbon reservoirs. Nevertheless, many of the principles discussed below are also applicable to and relevant for clastic aquifers and reservoirs, and the arguments are made to include both carbonates and clastics whenever appropriate and/ or possible. This is especially true for the classification of diagenetic settings proposed below, which is generally applicable. For a more specific discussion of the effects of groundwater flow on clastic aquifers, the reader is referred to Schwartz and Longstaffe (1988). As for aquitards, in the present context it is sufficient to recognize that they also experience diagenetic changes (particularly regarding their clay mineralogy). These changes are of relatively minor importance for water and hydrocarbon supply, however, because fluxes through aquitards are, by definition, very low. Consequently, neither porosity nor permeability is significantly affected by diagenetic changes in aquitards, except during the first km of burial, when clayrich sediments and “soon-to-be aquitards” experience significant physical compaction, clay authigenesis, and concurrent porosity loss.

Diagenetic Settings The realm of diagenesis comprises all mineralogical, physical, and chemical changes of sedimentary deposits between the time of ultimate deposition and the onset of metamorphism (greenshist facies). Diagenetic settings are commonly divided into “shallow,” “intermediate,” and “deep” burial, yet these terms are ill defined and hydrological/hydrogeochemical criteria are usually ignored (a noteworthy exception is Galloway and Hobday 1983). Three main reasons exist for the relatively poor definition of diagenetic burial settings: (1) some diagenetic processes, such as bacterial sulfate reduction and microbial methanogenesis, commonly take place at depths of only a few cm to m in “immature,” subsiding basins with active sedimentary deposition (e.g., Berner 1980), whereas the same processes may also take place at hundreds of meters of depth (common maximum of about 2000 m) in “mature,” Hydrogeology Journal (1999) 7 : 94–107

subaerially exposed and depositionally inactive basins (e.g., Machel and Foght, in press); (2) some diagenetic processes, such as thermochemical sulfate reduction, take place only at relatively great depths and temperatures, regardless of whether the basin is immature or mature (with the exception of shallow settings that are hydrothermal and/or have very high heat flow); and (3) some diagenetic processes, such as stylolitization, do not require externally imposed pore-water fluid flow. Hence, a distinction must be made for at least some diagenetic processes and effects as to whether the basin is an immature subsiding basin with active deposition, or a mature exposed basin with insignificant deposition and/or erosion. It is herewith recommended to combine mineralogic, geochemical, and hydrogeologic criteria from clastics and carbonates, the occurrence of hydrocarbons, and fractures, into a comprehensive classification of diagenetic settings that is generally applicable. This classification includes near-surface, shallow-, intermediate-, and deep-burial diagenetic settings, hydrocarbon-contaminated plumes, and fractures, as shown in Figure 1. Within limits, this new classification is compatible with less comprehensive or less general classifications; examples are shown in Figure 2.

Near-Surface Diagenetic Settings Near-surface diagenetic settings are herewith defined as those within a few meters of burial, where the pore fluids are surface-derived and essentially unaltered meteoric, brackish, marine, or evaporitic. Diagenetic case studies may specify whether the basin is immature or mature. Marine diagenetic settings, which can be subdivided into various subsettings (e.g., James and Choquette 1990), are included here because of the significant marine diagenesis many carbonates experience before they are affected by groundwater. Marine diagenesis is very subdued, if not negligible, in most clastics. The hydrologic drives vary from place to place and include wind action, wave action, and tidal pumping in marine settings; density-driven reflux in evaporitic settings; and gravity drive in vadose/meteoric settings.

Shallow-Burial Diagenetic Settings Shallow-burial diagenetic settings (also called shallow subsurface settings/ environments) are similar in many respects, including hydrogeochemistry, to the nearsurface diagenetic settings. Important differences include added physical compaction and hydrologic conditions that may vary from place to place. As in the case of near-surface settings, diagenetic case studies may specify whether the basin is immature or mature. Common textures generated by physical compaction in carbonates and/or clastics include thinning of laminae between, and draping over, concretions; flatQ Springer-Verlag

97 Figure 1 Classification of diagenetic settings on the basis of mineralogy, petroleum, hydrogeochemistry, and hydrogeology. For illustrative simplicity, the geologic section is assumed to be isotropic and homogeneous, with idealized groundwater flow lines. The hydrocarbon-contaminated plume is slightly deflected by the local and regional groundwater flow systems. The depth limits separating the burial diagenetic settings are approximate and based on geologic phenomena that are easily recognizable. Near-surface settings may be meteoric, brackish, marine, or hypersaline

tened to squashed burrows, fenestrae, gas-escape structures, desiccation cracks, and grains; spalling of coated grains; swirling structures; telescoping (conversion of grain-poor to grain-supported textures); and planar to curviplanar grain contacts (e.g., Machel 1990). In continental or island settings, the diagenetic pore waters are oxygenated, surface-derived groundwaters that have experienced little or no water–rock interaction. The maximum penetration depth of dissolved oxygen commonly is around 600–1000 m (Andreev et al. 1968; Krouse 1983). In addition, 300–600 m is the interval where chemical compaction (pressure solution) commonly commences in carbonate rocks, with recognizable stylolites developing at depths below about

600 m, the depth at which stylolites begin to form even in pressure solution-resistant chalks (e.g., Lind 1993). In clastic rocks, stylolites can be formed at approximately the same depths between carbonate grains or cements. Combining these arguments, the lower boundary of shallow-burial settings is herein defined to be located at depths of approximately 600–1000 m. Hence, as a generalization, sediments and rocks that have a predominantly oxidized mineralogy (e.g., Fe 3c and Mn 4c in solid solution or as admixed minerals), and where interbedded carbonates display evidence of physical compaction but no or poorly developed stylolites, are defined to have undergone shallow-burial diagenesis.

Figure 2 Comparison of popular previous classifications of diagenetic burial settings with the one proposed in this study. The previous classifications were established on the basis of organic maturation (Bustin et al. 1985), carbonate diagenesis (Choquette and James 1990), and clastic diagenesis, including hydrogeochemical and hydrologic criteria (Galloway and Hobday 1983). Ropvitrinite reflectance

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Implicit in the above definition of shallow-burial settings are certain hydrological conditions that vary with the (paleo-)geographic setting. In marine settings of immature basins, much if not most pore-water flow at very shallow depths is facilitated by tidal pumping, giving way to compaction with increasing depth. In coastal mixing zones, pore-water throughput is facilitated by the convergence of two forces, i.e., gravity (topography) drive from the meteoric water and density drive from the seawater. In evaporitic settings, most fluid throughput is via density-driven reflux. Subaerially exposed settings are usually governed by gravity drive in local groundwater flow systems (sensu Tóth 1963) that respond to surface conditions such as weather, seasons, and latitude. An important exception, from a hydrogeochemical point of view, is places where local and regional groundwater discharge areas merge, and where deep(er) groundwaters mix with shallow groundwaters, providing for special hydrogeochemical settings.

Intermediate- and Deep-Burial Diagenetic Settings Intermediate- and deep-burial diagenetic settings are located below shallow burial settings. In immature basins, the predominant hydrologic drive is compaction. In mature basins, intermediate- and deep-burial diagenetic settings are located within intermediate or regional groundwater flow systems (sensu Tóth 1963; Lovley and Chapelle 1995). Rocks in intermediate-burial settings experience chemical compaction as well as subsurface cementation and dissolution. Pressure solution textures in carbonates are diverse and include stylolites (one or several generations), other types of sutured seams, isolated or fitting nodules, non-sutured seams, and pseudobedding (Bathurst 1987; Choquette and James 1987). Most burial cements in carbonate rocks are calcite, dolomite, and anhydrite. Burial cements have various characteristics (e.g., Machel 1990): (1) crystals crosscut stylolites, or microstylolites and pressure solution seams terminate at cement crystals; (2) crystals occur in fractures or spalled ooid cortices; (3) crystals fill compacted pores; (4) crystals enclose compacted grains; (5) crystals may contain hydrocarbons in fluid inclusions; (6) crystals partially replace shallower burial cements; (7) crystals have relatively large size (diameters are greater than several tens of micrometers up to several centimeters); (8) crystals are blocky, form equant mosaics, increase in mean diameter toward pore centers, or are poikilotopic; (9) crystals are not recrystallized; (10) crystals show well-defined cathodoluminescence zonation from bright towards quenched (dull) luminescence, or they are dull-luminescent throughout, commonly caused by ferroan composition (divalent iron 1 500 ppm); (11) crystals have oxygen isotope ratios that are depleted relative to marine cements of the same stratigraphic age as the host rocks; and (12) crystals contain two-phase fluid inclusions Hydrogeology Journal (1999) 7 : 94–107

with elevated temperatures of homogenization ( 1 50 7C) and large freezing-point depressions (indicating high salinities), which can be used to interpret the hydrogeochemical environment of formation. The lower limit of intermediate-burial diagenetic settings, and thereby the upper limit of deep-burial diagenetic settings, is herein defined relative to the top of the liquid oil window in hydrocarbon source rocks. This depth offers another natural defining limit and has indeed been used to define the boundary between “diagenesis” and “catagenesis” in petroleum geology (e.g., Bustin et al. 1985; Hunt 1996). In mineral diagenesis, this boundary is useful because the introduction of oil into the pore spaces commonly arrests diagenesis. Unfortunately, the depth of the top of the oil window varies widely, depending on kerogen type and geothermal history, with a distribution maximum of about 2000–3000 m (Hunt 1996). This depth interval is herewith taken as the bottom of intermediate-burial diagenetic settings. Deep-burial diagenetic settings merge into the metamorphic realm at temperatures around 200 7C and commensurate depths and pressures that depend on the geothermal gradient (e.g., Winkler 1979), i.e., about 6 km and 150 MPa (1.5 kbars) at 30 7C/km, or about 9 km and 225 MPa (2.25 kbars) at 20 7C/km. Semantic confusion and overlap exist regarding deep-burial diagenesis and incipient metamorphism, a realm to which different terms have been applied by various authors: “very low grade metamorphism,” “anchimetamorphism,” “late diagenesis,” “burial metamorphism,” “catagenesis,” “epigenesis,” “metagenesis,” and “epigenetic diagenesis” (Frey 1987, p. 4). The vast majority of sedimentary rocks spend most of their existence in the intermediate- and deep-burial realms, where porosity and permeability are often altered significantly, and where hydrocarbon maturation, migration, and most trapping occurs. The associated groundwaters (also called burial fluids, formation fluids, subsurface brines, etc.) are governed in their composition largely or entirely by water–rock interaction.

Hydrocarbon-Contaminated Plumes A special type of diagenetic setting that may crosscut all settings defined above are hydrocarbon-contaminated plumes that originate from leaking oil and gas traps (Figure 1). The escaping hydrocarbons tend to form subvertical plumes of moderately to strongly reducing conditions. The plumes tend to undergo diagenetic reactions that are absent, or at least much less pronounced, outside the plumes, such as the formation of magnetic mineral assemblages, of carbonate cements with distinctive cathodoluminescence and/or isotopic composition, soil-gas anomalies, distinctive vegetation, and a number of other phenomena (Barker et al. 1991; Burton et al. 1993; Machel 1995; Tedesco 1995; Schumacher and Abrams 1996). Q Springer-Verlag

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Fractures Another special type of diagenetic setting is fractures that may be restricted to one of the diagenetic zones defined above or may transgress two or more of these zones (Figure 1). In the first case, diagenesis within the fractures is generally similar to that within the adjacent pore network. In the latter case, diagenesis within the fractures may be significantly different from that in the surrounding wall rocks, depending on how fast the waters/fluids move through the fractures, and whether these waters/fluids are significantly warmer, colder, underpressured, or overpressured relative to those in the wall rocks (further discussed below). Temporal Designations In paragenetic sequences, which list diagenetic processes in the chronological order that they affected the rock(s) unit under investigation, diagenetic processes are often given temporal designations, such as “early” or “late.” However, such temporal designations are only relative and usually meaningless hydrogeochemically. For example, a “late-diagenetic” 16th phase in a complicated series of 18 phases may have formed in a near-surface setting from meteoric water perhaps only a few tens to hundreds of thousands of years after deposition, which is not uncommon in Cenozoic carbonates (e.g., Schroeder and Purser 1986, Section I). Alternatively, a relatively “early-diagenetic” 5th phase of a total of 18 phases may have formed at a depth of nearly 3 km some 300 m.y. after deposition, which is common in Paleozoic carbonates where the truly earlydiagenetic phases, i.e., those that formed during shallow burial, are obliterated and/or rendered unrecognizable by pervasive recrystallization (e.g., Schroeder and Purser 1986, Section III). Hence, temporal designations should be used only in conjunction with the (hydrogeochemical) burial settings proposed above.

Intrinsic and Extrinsic Factors Diagenetic processes that affect porosity and permeability in any type of sedimentary rock are cementation, dissolution, compaction, recrystallization, replacement (in carbonates, predominantly dolomitization), and sulfate-hydrocarbon redox-reactions. To be volumetrically significant, processes such as cementation, dissolution, and dolomitization require significant flow of groundwater driven by an externally imposed hydraulic gradient, because of the relatively low aqueous solubilities of almost all minerals. The other processes can and often do take place without significant groundwater flow, or even in hydrologically stagnant, geochemically “closed” systems (e.g., Machel 1990). Physical compaction is intermediate between these two groups. Physical compaction is not stagnant, as it generates flow, but it does not require an externally imposed flow. Hydrogeology Journal (1999) 7 : 94–107

The above diagenetic processes are governed by several factors that affect individual rocks as well as any rock sequence in a sedimentary basin as a whole: increasing temperature and pressure with depth, chemical changes in the pore fluids, and various types of groundwater flow (e.g., Hanor 1987, 1994; several articles in this volume). The rocks influence, and are affected by, these factors, an interplay that is summarily called “water–rock interaction.” Furthermore, as far as these factors are concerned, diagenesis of one rock type cannot be separated from the diagenesis of other rock types, especially in intermediate- to deep-burial settings, where intercommunication of all rock types by groundwater flow is common; such conditions are illustrated in Figure 3. Nowhere is this more obvious than in the case of subsurface dissolution. Whereas many carbonates have secondary porosity from dissolution in diagenetic burial settings, most processes that generate acids originate in, or are facilitated by, clastic and carbonaceous rocks (e.g., Giles and Marshall 1986; Surdam et al. 1989). This is one reason why this article is geared toward the effects of groundwater flow on mineral diagenesis.

Thermodynamic Constraints Cementation and dissolution are governed mainly by the thermodynamic saturation state that is commonly expressed as saturation index SI (V by some authors), which is the log of the ratio of the ion-activity product in solution (Q) over the thermodynamic equilibrium constant (K, or K7 by some authors): SIplog (Q/K). Where a mineral occurs mainly as a replacement rather than as a passive pore-filling cement, thermodynamic stability is often expressed relative to the two minerals in question, e.g., calcite vs dolomite in temperature vs Ca/Mg-space for dolomitization (e.g., Carpenter 1980). In general, temperature, pressure, and composition of the groundwater determine the saturation state(s) and, hence, precipitation and dissolution, either directly or indirectly in one or several of the following ways. 1. The values of K of carbonates decrease with increasing temperature (retrograde solubility). In the case of calcite, for example, the decrease in solubility is about two orders of magnitude between 25 7C and 200 7C (e.g., Robie et al. 1979). Decreasing K results in increasing SI. Therefore, pore fluids in sedimentary basins, if buried and not changed significantly through water–rock interaction or mixing, become supersaturated with respect to carbonates with depth, even if the concentrations of the cations (calcium, magnesium, iron, etc.) and carbonate ions remain essentially unchanged. Conversely, ascending formation/groundwaters that cool tend to become undersaturated with respect to carbonates, with the potential to generate significant secondary porosity in carbonates and/or clastics that contain carbonate cements (e.g., Giles and deBoer 1989). Q Springer-Verlag

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Figure 3 Hydrogeologic section of the south Louisiana Gulf Coast, showing the distribution of major flow regimes. [Reproduced from Morse et al. (1997) with permission]

Carbonates and sulfates are the only common rockforming minerals with retrograde aqueous solubilities. Almost all other rock-forming minerals have prograde aqueous solubilities, i.e., their solubilities increase with increasing temperature. Obviously, the above arguments regarding ascending or descending pore waters have to be reversed for minerals with prograde solubilities. 2. The concentrations of the cations and carbonate ions in solution often increase due to pressure solution and/or a variety of diagenetic reactions involving non-carbonate minerals, such as silicates and sulfates. A common result is that ion-activity products (Q) for carbonates in solution increase, which increases SI, often leading to supersaturation and to precipitation of carbonates (e.g., Hanor 1987). 3. The saturation indices of carbonates are related to acidity, because an increase in acidity of the solution lowers Q through a decrease in CO3 2–, thereby decreasing SI. An increase in acidity can take place in many ways, including meteoric water penetration, mixing of waters, carbon dioxide generation from maturation of organic matter, generation of organic acids, and formation of acids from clay mineral reacHydrogeology Journal (1999) 7 : 94–107

tions (e.g., Giles and Marshall 1986), leading to carbonate dissolution. 4. Thermodynamic calculations indicate that mixing of groundwaters may lead to pronounced undersaturation or supersaturation with respect to carbonate minerals, depending on temperature, pressure, and fluid composition, particularly pCO2. As a generalization, mixing of shallow groundwaters at relatively low temperatures commonly results in undersaturation and dissolution (“Mischungskorrosion” sensu Bögli 1964; Plummer 1975), which is well known from karst terrains (e.g., Bonacci 1987; James and Choquette 1987) and from caves in coastal seawater/ fresh-water mixing zones (e.g., Machel and Mountjoy 1990). On the other hand, mixing of fresh groundwaters with brines in deep burial settings commonly leads to supersaturation and carbonate precipitation (Morse et al. 1997). 5. Pressure decrease, with or without a significant temperature decrease, almost invariably leads to carbonate supersaturation and precipitation. The most important parameter governing the degree of supersaturation is pCO2, and thermodynamic calculations show unexpected non-linear saturation profiles with depth (Morse et al. 1997). Under the assumption that minerals and solutions attain local thermodynamic equilibrium along the flow path, the above processes can be quantified in computer simulations to predict the precipitation and Q Springer-Verlag

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dissolution of diagenetic minerals along the flow path in basin strata as groundwaters migrate along temperature and pressure gradients. Using this approach, rock porosity evolution can be calculated from relative amounts of mineral precipitation and dissolution (e.g., Lee 1997; Morse et al. 1997). Permeability changes, however, can merely be inferred, as pore throat sizes and tortuosity cannot be predicted from thermodynamics.

Diagenetic Potential and Kinetic Factors Carbonates, like clastics, have different diagenetic potential during burial, i.e., in any diagenetic setting certain minerals or types of grains exist that dissolve, form, or recrystallize preferentially over others. Such phenomena are governed by textural and/or kinetic factors. One of the best known aspects is that small grains dissolve preferentially and more rapidly than larger grains, e.g., during recrystallization and/or Ostwald ripening (e.g., Folk 1965; Morse and Casey 1988). An example of microstructural control is that aragonite with complex microstructures can dissolve more rapidly than thermodynamically less stable magnesian calcite (e.g., Walter 1985). Kinetic factors overriding thermodynamic constraints during precipitation are also well known, particularly from near-surface and shallow diagenetic settings. Examples are the wellknown kinetic inhibition of dolomite formation from seawater, and the formation of ’protodolomite’ rather than dolomite from evaporated groundwater or seawater (e.g., Machel and Mountjoy 1986; Usdowski 1994). Such kinetic problems are ignored in the remainder of this paper, however, under the assumption that most diagenetic settings have had enough time to adjust to a thermodynamic equilibrium mineral assemblage. This certainly is true for most interme-

diate- and deep-burial settings, where sudden incursions of “alien” formation fluids are relatively rare and flow rates are generally low, except in fracture systems. Another factor that is important, especially in intermediate- and deep-burial settings, is that hydrocarbons often arrest diagenesis, either because they coat potentially reactive grain surfaces in oil-wet systems, or because the relative permeability to water is zero in the case of high oil saturation in water-wet systems (e.g., Laudon 1996). In both cases, ionic exchange through the aqueous phase (dissolution, precipitation), and hence diagenesis, is inhibited, except for local and volumetrically insignificant mineral redistribution by diffusion. However, some hydrocarbons may react with the rocks, as in the case of thermochemical sulfate reduction, whereby H2S, CO2/HCO3 –, and oxidized hydrocarbons are generated as the major reaction products (e.g., Machel et al. 1995).

Evolution of Porosity and Permeability with Increasing Burial in Various Hydrogeochemical Regimes The intrinsic and extrinsic factors discussed in the previous section require and/or interact with and/or are mediated by groundwater flow. The major effects are porosity and permeability creation, preservation, or reduction relative to the amounts of primary porosity and permeability, which are controlled by depositional environment and facies. Figure 4 illustrates these factors and their effects, including porosity loss with depth, for the well-studied limestones and dolostones from Florida, USA and the Smackover oolite reservoirs in the southern United States. Even though the trend is generally toward porosity reduction with depth, some

Figure 4 Major processes of porosity and permeability (“poroperm”) generation, preservation, and reduction in carbonates. The inset contains averaged porosity/ depth data from limestones and dolostones in south Florida, USA (stippled trend, from Schmoker and Halley, 1992) and Smackover oolite carbonate reservoirs in the southern United States (solid trends, which envelope the measured maximum and minimum values below depths of about 1.5 km, from Scholle and Halley 1985, and Heydari 1997). The Florida trend can be considered typical for most carbonates elsewhere. The large variations in Smackover carbonates at any given depth reflect highly variable degrees of porosity generation, preservation, and reduction due to various competing diagenetic processes Hydrogeology Journal (1999) 7 : 94–107

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Smackover rocks have much higher, others much lower, porosities than the average. Permeabilities are assumed to decrease with decreasing porosities (comparable permeability/depth data sets are not available). The evolution of porosity and permeability with depth in these rocks, which are typical for many, if not most, carbonate rocks elsewhere, apparently can be highly variable, depending on the relative importance of the various processes involved. These processes begin syndepositionally.

Depositional Controls In carbonates, much more so than in clastics, depositional environment and facies, especially cycle stacking patterns, play an important role in determining the porosity and permeability distribution and their subsequent evolution through burial diagenesis. A case in point is the Knox carbonates in Tennessee, USA (Montan˜ez 1997). Transgressive cycles, which originally were fairly porous and permeable, now consist of limestones and ~10 to 1 50 vol-% “late-diagenetic” dolomite with an average of 8.2% porosity and average permeability of 70.2 md, interpreted to be formed during intermediate to deep burial. Petrographic data also show that these rocks were affected by extensive dissolution in intermediate- to deep-burial settings. On the other hand, the facies within regressive cycles are almost completely replaced by tight fine-crystalline dolomite that formed syndepositionally from evaporated seawater. The transgressive carbonate cycles behaved like aquifers during later burial diagenesis, whereas the regressive cycles became aquitards almost syndepositionally. Other examples for depositional control on diagenesis are reef and reef-margin carbonates, which often are much more porous and permeable than back-reef and deeper fore-reef carbonates (e.g., Schneidermann and Harris 1985; Schroeder and Purser 1986). This often leads to a pronounced differentiation into “proto-aquifers” and “proto-aquitards” at the time of deposition.

hydrologically less vigorous settings. Recrystallization of the metastable carbonates aragonite and magnesian calcite also occurs but generally has little effect on porosity/permeability development.

Coastal mixing-zone diagenesis

In coastal areas, where seawater mixes with fresh water, the predominant diagenetic process affecting carbonate sediments and rocks is dissolution (Plummer 1975; Machel and Mountjoy 1990). Groundwater fluxes in coastal mixing zones (also called “zones of dispersion”) typically are quite high, and the effects are enhanced porosity and permeability, including caves, which are well known from many such settings. Other well known processes in coastal mixing zones are dolomite cementation and/or replacive dolomitization. However, the effects of these processes are volumetrically insignificant (less than 1 vol-%) in almost all cases (Machel and Mountjoy 1990).

Meteoric diagenesis

Meteoric waters in near-surface settings are almost always undersaturated with respect to carbonates, because rain water is essentially devoid of earth alkali metals yet slightly acidic because of dissolved atmospheric CO2. Where the ground has a significant soil cover, pCO2 in the downward percolating rain water of the vadose zone can easily increase by two orders of magnitude because of extensive plant and microbial activity in the soil zone. This increase results in extensive dissolution in the upper few meters of burial, and the effects are increased porosity and permeability and/ or physical weakness of the rocks. In the Caribbean islands, for example, many of which have a cover of Cenozoic carbonates that have not been buried by more than a few meters, the carbonates are frail and crumbly because of a lack of near-surface cementation and/or subsequent meteoric dissolution (e.g., James and Choquette 1984).

Near-Surface Settings Marine diagenesis

Hypersaline diagenesis

Most carbonate rocks begin their lives as marine sediments with primary porosities around 40–45%. Hence, the first pore water to affect carbonate sediments is seawater, and the diagenetic setting is near-surface marine, where the predominant hydrologic drives are wind/wave action and tidal pumping. The main effects on the sediments and rocks are partial to extensive filling of primary pores with internal sediments and marine carbonate cementation (e.g., Tucker 1990), which leads to significant porosity reduction (Figure 4). In general, highly porous and permeable facies types affected by vigorous waves and tides tend to become more cemented than the other facies types and/or

The only volumetrically important and widespread diagenetic effect of groundwater flow on carbonates in near-surface evaporitic settings is sabkha-type dolomitization. Typically, fresh groundwater and/or seawater that are evaporated to or beyond gypsum saturation flow seaward within the uppermost few meters of the carbonate sediments via density-drive. In the process, they form fine-crystalline and geochemically distinct dolomite, either as a replacement or as a cement. The resulting dolostones typically are laterally extensive but thin aquitards that may form important seals for hydrocarbon traps (e.g., Machel and Mountjoy 1986; Moore and Wilde 1986).

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Shallow-Burial Settings The four near-surface hydrogeochemical settings discussed above (marine, brackish, meteoric, and hypersaline) often persist into shallow-burial diagenetic settings. Significant additional processes are physical compaction and, at greater depths, incipient chemical compaction, as well as a relatively minor amount of water–rock interaction. The major differences compared to near-surface settings are that deeper meteoric settings often experience cementation and porosity/permeability loss rather than dissolution and porosity/permeability gain. Effects of physical compaction

With increasing burial, physical (mechanical) compaction leads to loss of water and porosity. Carbonate sediments can compact to about one–half of their original thickness under as little as 100 m of overburden by physical compaction alone (Choquette and James 1987). Thereby, sedimentary particles and structures are modified and rearranged until a self–supporting framework is achieved at an average porosity of about 40%. Further burial will lead to grain deformation in the form of ductile (squeezing) or brittle (breakage) failure, followed by chemical compaction. In contrast to most other diagenetic processes and effects, the extent of physical compaction and concurrent porosity/permeability loss strongly depends on depositional setting and near-surface diagenesis. More specifically, shallow-water carbonates often become strongly cemented and solidified in the near-surface marine setting, which inhibits later physical compaction. On the other hand, cementation is often poor in deep-marine carbonates, such as chalks, which permits much more extensive physical compaction during burial and/or leads to significant porosity preservation (e.g., Choquette and James 1987; Maliva and Dickson 1992).

Intermediate- to Deep-Burial Settings Effects of chemical compaction

Chemical compaction (pressure solution) begins during the latter phases of physical compaction and continues into the metamorphic realm (Choquette and James 1987). Chemical compaction is most active in rocks that are being buried (as opposed to being stationary at some depth), and that have some porosity left to facilitate relatively uninhibited grain interpenetration and ionic movement from the points of dissolution into the pore fluids. An externally imposed hydraulic gradient is not necessary, however. The liberated ions move, via advection or diffusion, to areas of lower stress, either just adjacent to the area of dissolution or elsewhere in the pore system. Here, the dissolved material may reprecipitate, forming burial-diagenetic cements. Chemical compaction is known to result in thickness Hydrogeology Journal (1999) 7 : 94–107

reductions of at least 25–35%, in addition to the thickness losses caused by physical compaction. In Smackover oolites, which were used to construct the porosity/ depth curves in Figure 4, physical and chemical compaction combined account for a loss of 33% of originally 45% of porosity (Heydari 1997). Chemical compaction generally is detrimental for groundwater development and/or petroleum production. The major effects are porosity and permeability reduction with progressive chemical compaction as a result of the overall thickness reduction and reprecipitation of the material dissolved at grain contacts. Effects of cementation

Cementation, and hence porosity and permeability reduction, is common in intermediate- and deep-burial settings because of temperature increase, mixing, and chemical compaction (see above). Considering further that the solubility of most carbonates is rather low (for example, Kp10 –8.52 at 25 7C for calcite, decreasing with increasing temperature; Robie et al. 1979), cementation of about several vol-%, which is common (Figure 4), requires large fluxes. Numerous case studies have demonstrated extensive intermediate- and deep-burial cementation (and concurrent porosity/permeability losses) in carbonate aquifers, not only with carbonate minerals but also with sulfates, sulfides, and clay minerals (e.g., Scholle and Halley 1985; Walls and Burrowes 1985; Woronik and Land 1985; and several articles in Montan˜ez et al. 1997). Some case studies, however, have shown deep burial cementation to be relatively minor (e.g., Prezbindowski 1985). Effects of dissolution

Dissolution, albeit volumetrically not as important as cementation in intermediate- and deep-burial settings (or else a general porosity decrease with depth would not occur), nevertheless is common, especially near the oil window when decarboxylation and certain other mineral reactions generate acidity. Secondary porosity is well known from carbonates (Choquette and Pray 1970) and clastics (Schmidt and MacDonald 1979). Significant dissolution may or may not involve large fluxes. If acidity is generated within or near the rock with secondary porosity, large-scale groundwater flow is not necessary. A hitherto controversial process to generate secondary porosity in deep burial settings is thermochemical sulfate reduction (TSR). Model calculations suggest that partial dissolution of carbonate rocks may occur at least initially during TSR (e.g., Hutcheon 1992; Nicholson 1992), but most published case studies do not show a significant increase in porosity (e.g., Machel et al. 1995). TSR would rank among the processes of in situ dissolution, because these processes commonly take place in hydrologically closed or nearly closed systems (e.g., Machel et al. 1995; Machel 1997). Q Springer-Verlag

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Effects of recrystallization

Due to the increasing temperature, pressure, and changing composition of the groundwaters, the matrix, allochems, and cements formed at shallower depths generally become thermodynamically metastable or unstable with increasing burial. Barring kinetic inhibition, recrystallization and replacement of metastable and unstable minerals will occur. Porosity and permeability are generally little affected by recrystallization.

Effects of replacement

Next to dolomitization (discussed separately below), the most common type of replacement in deep carbonate aquifers is by anhydrite. For example, extensive anhydrite replacement of dolostones, with or without concomitant anhydrite cementation, is well known from the Devonian strata of western Canada (e.g., Walls and Burrowes 1985; Machel 1990). This process can have a pronounced effect on porosity and permeability. Regardless of the actual quantities of anhydrite formed relative to dolomite replaced or cemented, dolostones commonly are porous, whereas the resulting anhydrite is very tight. Hence, even if relatively small amounts of anhydrite are formed at the expense of dolomite, the result often is a marked reduction of the permeability of the carbonate aquifer.

Effects of epimetamorphic fluids

One poorly understood phenomenon is diagenesis in the deepest parts of the diagenetic realm that grade into the epimetamorphic realm. Some authors (Land 1997; Schroyen and Muchez 1998) recently advocated that material transfer may happen from the crystalline basement into the overlying deep burial diagenetic setting, whereby prograde metamorphism and devolatilization reactions liberate water and CO2, which are added to the sedimentary basin above. It is open to speculation at this time whether “injection” of metamorphic fluids into the aquifers above occurs to any significant degree, and what effects this type of groundwater flow may have on deep-burial diagenesis.

Fractures One phenomenon that transgresses all zones within the diagenetic realm is mineral diagenesis in fractures. Where fractures are restricted in lateral or horizontal extent within one of the above diagenetic burial zones, the diagenesis within the fractures is generally very similar or identical to that within the surrounding wall rocks. Where fractures cross from one diagenetic zone into another, however, relatively cold groundwaters may descend or relatively hot groundwaters may ascend fairly rapidly. Hence, the fractures in any diagenetic zone may contain undercooled (and underpressured) or hydrothermal (and overpressured) fluids relative to those in the surrounding wall rocks. This situaHydrogeology Journal (1999) 7 : 94–107

tion raises numerous possibilities for mineral reactions, especially when the fractures transect different lithologies (e.g., Pedersen and Bjørlykke 1994; Stoute and Harris 1995). A comprehensive discussion of these possibilities is beyond the scope of this paper. The following examples characterize the most common and perhaps (for humans) most important aspects of groundwater flow though fractures. Rapid descent of cool meteoric waters to depths of several km through deep-reaching faults/fractures is common in mountainous recharge areas. In the Canadian Rockies, for example, such hydrologic systems are common in the Front Ranges, where meteoric waters convectively descend and return to the surface in sulfurous hydrothermal springs, e.g., at Miette, Banff, and Radium. The diagenetic effects of “groundwater flow” in the descending parts of these flow systems are virtually unknown, except that the waters appear to equilibrate isotopically rather fast in at least some locations (e.g., Nesbitt and Muehlenbachs 1997). The major effect in the ascending parts of these flow systems is massive cementation within the fractures, which are commonly filled with calcite, dolomite, quartz, and to a minor degree with other minerals, including barite and fluorite (Nesbitt and Muehlenbachs 1997). Similarly, from a hydrogeological point of view, rapid convection of meteoric waters is known from intermontane rift basins. In one of the best-studied examples, Tóth and Otto (1994) demonstrate that meteoric waters move preferentially through fractures from recharge areas in the mountains that fringe the Rhine Graben toward the enclosed valley, where they discharge as hydrothermal and petroliferous brines. Radiocarbon data of waters from producing wells range from 3,165–31,000 yr, indicating that convection is very rapid indeed. The diagenesis within the fractures has not been investigated, however. Rapid ascent of hydrothermal fluids derived from shallow-metamorphic and deep-burial diagenetic settings is well known from many sedimentary basins. Perhaps the best known effect of such “hydrothermal groundwater flow” is Mississippi-Valley-Type Pb-Zn (saddle) dolomite mineralization, which may be restricted to the fractures or may partially replace the wall rocks (e.g., Anderson and Macqueen 1982). Fractures commonly serve as migration pathways for petroliferous fluids. In such cases, the fluids may form minerals with petroliferous fluid inclusions and/or solid bitumens in the fractures, which can be used to determine hydrocarbon migration pathways and related aspects, such as timing of migration and fluid composition (e.g., Levine et al. 1991).

Porosity and Permeability Preservation Another phenomenon that transgresses all zones within the diagenetic realm is porosity and permeability preservation. Feazel and Schatzinger (1985) identify the following processes as those that, in combination, Q Springer-Verlag

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tend to preserve porosity: (1) shallow (minimal) burial; (2) reduced burial stress, generally due to overpressured pore fluids; (3) increased framework rigidity due to near-surface lithification; (4) permeability barriers, isolating porous rock intervals or domains from supersaturated groundwaters; (5) exclusion of pore waters by petroleum entry; and (6) low diagenetic potential due to a stable mineralogy (see also Figure 4, where the processes are listed for poroperm preservation). Processes 1–3 tend to minimize or inhibit physical and chemical compaction. Processes 4–6 prohibit significant fluxes. Porosity, whatever its origin, would be preserved.

Dolomitization One of the most important effects of groundwater flow on carbonate aquifers is pervasive replacive dolomite formation, i.e., dolomitization, with a concomitantincrease in porosity and permeability. Mass-balance calculations indicate that large fluxes are necessary for extensive, pervasive dolomitization, because of the generally low Mg content of natural waters. About 6,500–10,000 m 3 of brackish-to-fresh groundwaters (the former containing about 10% seawater) would be required to dolomitize one m 3 of limestone with an initial porosity of 40%; even if seawater, the most common Mg-rich water in nature, is used for dolomitization, about 650 m 3 would be required per m 3 of limestone (e.g., Machel and Mountjoy 1986). The net porosity increase results from the fact that the reaction stoichiometry commonly is about 2 moles of calcite being replaced by 1 mole of dolomite, with a concomitant decrease in the molar volume by about 13%. Other reaction stoichiometries, however, may result in little porosity change or in porosity loss during dolomitization (e.g., Machel and Mountjoy 1986). Massive dolostones with enhanced porosity and permeability relative to their limestone precursors are common all over the world, and popular dolomitization models typically depict limestones embedded in some type of local, intermediate, or regional groundwater flow system that is invoked to pump the necessary amounts of Mg through the rocks (e.g., Amthor et al. 1993; various articles in Purser et al. 1994; and Mountjoy et al. 1997). Thereby, the distribution, texture, and geochemical composition of the dolostones is fitted to perceived or possible models such as those shown in Figure 5, in an attempt to obtain a viable genetic interpretation. Such models span all diagenetic settings from near-surface to the metamorphic realm, as well as almost any type of hydrologic flow system and geochemical composition.

Conclusions The effects of groundwater flow on mineral diagenesis, especially in carbonate rocks, are manifold. Although Hydrogeology Journal (1999) 7 : 94–107

Figure 5 Diagrams of several groundwater flow systems (left) and predicted dolomitization patterns (right). Examples are of incomplete dolomitization of carbonate platforms or reefs. Arrows denote flow directions; dashed lines show isotherms. Predicted dolomitization patterns are shaded. Models A–E are km scale; model F is basin scale. (After Amthor et al. 1993)

porosity and permeability generally decrease with depth, many carbonates have highly variable poroperm values that may be much higher or lower than the average. The various processes of porosity generation, preservation, and reduction “compete” with one another, and the present porosity and permeability characteristics represent the net result of all involved processes. Considering the immense range of possibilities, generalized predictions of poroperm characteristics are risky, and each aquifer should be investigated on an individual basis. Acknowledgments The reviews by Ian Hutcheon, Andrea Mindszenty, and József Tóth are much appreciated. The research leading up to the synthesis presented in this paper was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). Q Springer-Verlag

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