Limestone and Dolomites Balrambhadu ONGC, Dehradun

Limestone and Dolomites

BalramBhadu ONGC, Dehradun

1.0 Introduction

Understanding of earth’s history is core business of geology and to understand it in totality requires inputs from other associated disciplines. Study of geology involves origin, composition and structure of the earth. Among the various disciplines of earth system sciences, sedimentology is an integral part to reveal earth’s history. Sedimentology involves study of sedimentary rocks and their formation through various geological processes covering weathering, erosion, transportation and deposition by geological agents like water, air, ice etc. Sedimentology also involves post depositional changes (diagenesis) which take place after deposition of sediments upto the stage of initiation of metamorphism. Sedimentology deals with all type of sediments whether recent or ancient through the geological periods deposited in wide range of depositional environments. By volume percentage, sedimentary rocks may be lesser in amount out of total rocks of the earth but they provide much of the world’s iron, potash, salt, building material and energy resources. Sedimentary rocks include-

a. Siliciclastic sedimentary rocks (epiclastic, detrital/ terrigenous): Formed by physical and chemical disintegration of pre-existing rocks like conglomerate, breccias, sandstone, siltstone, shale, and mudstone. These are also known as clastic sedimentary rocks. b. Biogenic, biochemical/ organic sedimentary rocks: These types of rocks are formed precipitation of minerals by the organisms like carbonates, chert, phosphates etc. These are also known as non-clastic sedimentary rocks. c. Chemical sedimentary rocks: Formed by precipitation of dissolved components in water like evaporites.These are also part of non-clastic sedimentary rocks.

Among the non-clastic sedimentary rocks, carbonates are the most abundant rocks constituting about 10% of total sedimentary rocks. These rocks are formed chiefly by chemical and biochemical processes. Moreover, biological and biochemical processes dominant the formation of carbonates

but inorganic precipitation of CaCO3 from sea water also known. After deposition, physical and chemical processes of diagenesis considerably modify the carbonate sediments (Tucker, 1991).

The carbonate rocks include the limestones made of calcium carbonate (CaCO3), and dolomites composed of the mineral dolomite (CaCO3.MgCO3). Some workers prefer to use the term ‘dolomite’ for the mineral and refer the rock as ‘dolostone’. Carbonates are important aquifers and hydrocarbon reservoirs owing of the high porosity which they sometimes contain (Selley, 2000). Carbonate rocks account for about 60% of hydrocarbon reserves of the world and notable hydrocarbon reservoirs in the Middle East (Saudi Arabia and Kazakhstan) and India (Mumbai Offshore) are found in carbonates. Limestones also host epigenetic lead and zinc sulphide deposits across the globe. Limestone is also important source of lime to make cement, and is hence a component of all concrete, brick and stone buildings and other large civil engineering structures such as bridges and dams.

Limestones are most common and widespread rocks that form the peaks of mountains in the Himalayas and also form characteristic karst landscapes and many spectacular gorges throughout the world. Limestone deposits are common through much of the stratigraphic record and include some very characteristic rock units, such as the Late Cretaceous Chalk, a relatively soft limestone that is found in many parts of the world. The origins of these rocks lie in a range of sedimentary environments: some form in continental settings, but the vast majority are the products of processes in shallow marine environments, where organisms play an important role in creating the sediment that ultimately forms limestones (Nichols, 2009). Difference between siliciclastic and carbonate sediments is tabulated here for basic understanding (Table 1).

Table 1: Difference between siliciclastic and carbonate sediments Carbonate Sediments Siliciclastic Sediments Mainly marine sediments Both marine and non-marine are common Grain size of sediments is generally controlled by size Reflects hydraulic energy in the of organism skeletons and calcified hard parts environment Presence of lime mud indicates rich growth of Presence of mud is an indicator of settling organisms through suspension medium Shallow water bodies reflect localized biological and Shallow water sand bodies are result of physiochemical changes interaction of current and waves Localized build up alter the character of surrounding Change in sedimentary environment sedimentary environment reflect change in hydraulic regime Main deposition in shallow tropical environment Sedimentation occur in all type of climate and environment Sedimentary facies features are obliterated even Sedimentary facies features are not during low grade metamorphism obliterated in low grade metamorphism

2.0 Carbonate Mineralogy and Chemistry

About 60 natural carbonate minerals are known and most common are calcite, aragonite and dolomite. Calcium carbonate (CaCO3) is the dominant constituent of modern carbonates and ancient limestones. It occurs as two minerals, aragonite and calcite. Aragonite crystallizes in the orthorhombic crystal system while calcite is rhombohedral. Calcite forms an isomorphous series with magnesite (MgCO3).

Ancient limestones are composed largely of low magnesium calcite, while modern carbonate sediments are made mainly of aragonite and high-magnesium calcite. Aragonite is found in many algae, lamellibranches, and bryozoa. highmagnesian calcite occurs in echinoids, crinoids, many foraminifera, and some algae, lamellibranches, and gastropods. Skeletal aragonite and calcite also contain minor amounts of strontium, iron, and other trace elements. The relationship between carbonate secreting organisms, mineralogy of their shells and trace elements are important because their variation and distribution play role in the early diagenesis of skeletal sands.

Dolomite is another important carbonate mineral, giving its name also to the rock. Dolomite is calcium magnesium carbonate (CaMg(CO3)2). Isomorphous substitution of some magnesium for iron is found in the mineral termed ferroan dolomite or ankeriteCa(MgFe)(CO3)2. Unlike calcite and aragonite, dolomite does not originate as skeletal material. Dolomite is generally found either crystalline, as secondary replacement of other carbonates or as a primary or penecontemporaneous replacement mineral in cryptocrystalline form.

Siderite,iron carbonate (FeCO3), is one of the rarer carbonate minerals. It occurs, apparently as a primary precipitate, in ooliths. These "spherosiderites," as they are termed, are found in rare restricted marine and freshwater environments. Spherosiderite is oftenassociated with the hydrated ferrous aluminosilicate, chamosite, in sedimentary iron ores. Siderite also occurs as thin bands and horizons of concretions inargillaceous deposits, especially in deltaic deposits. Siderite clasts are also found in intraformational conglomerates. These facts suggest that siderite forms diagenetic duringearly burial while the host sediment is still uncompacted. Its formation is favoured by alkaline reducing conditions. Figure1 shows different carbonate minerals stable at earth’s surface temperature and pressure (Selley, 2000).

Fig. 1: Composition triangle of carbonate minerals stable at surface temperature and pressure

Sedimentary rocks may also be made ofcarbonates of elements such as magnesium or iron,and there are also carbonates of elementsoccurring in nature (e.g. malachite and azurite arecopper carbonates). This group of sediments and rocksare collectively known as carbonates to sedimentarygeologists, and most carbonate rocks are sedimentaryin origin. Exceptions to this are marble, which is acarbonate rock recrystallised under metamorphicconditions, and carbonatite, an uncommon carbonate-rich lava.

Calcite

The most familiar and commonest carbonate minerals calcite (CaCO3). As a pure mineral it is colourless orwhite, and in the field it could be mistaken for quartz. It can be differentiated simply by hardness (3 onMohs’ scale) and reaction with dilute hydrochloric acid (HCl). Although calcite sometimesoccurs in its simple mineral form, it most commonly has a biogenic origin. A wide variety of organisms usecalcium carbonate to form skeletal structures and shells and calcareous sediments/ rocksare formed of material made in this way.Magnesium ions can substitute for calcium in thecrystal lattice of calcite, and two forms of calcite arerecognised in nature: low- magnesium calcite (low-Mgcalcite), which contains less than 4% Mg, and highmagnesiumcalcite (high-Mg calcite), which typicallycontains 11% to 19% Mg. The hard parts of manymarine

organisms are made of high-Mg calcite like echinoderms, foraminifers etc. Strontium may substitute for calcium in the lattice. It is in smallquantities (less than 1%) but important for strontium isotopestudies for dating of rocks.

Aragonite There is no chemical difference between calcite andaragonite, but the two minerals differ in theirmineral form: whereas calcite has a trigonal crystalform, aragonite has an orthorhombic crystal form.Aragonite has a more densely packed lattice structureand is slightly denser than calcite (specific gravity of2.95, as opposed to 2.72–2.94 for calcite),and is slightly harder (3.5–4 on Mohs’ scale). Many invertebratesuse aragonite to build their hard parts, includingbivalves and corals.

Dolomite Calcium magnesium carbonate (CaMg(CO3)2) i.e. dolomite is acommon rock-forming mineral. For the rock, term dolostone is also used for the lithology to distinguish it from dolomite mineral. The mineral is similar inappearance to calcite and aragonite, with a similarhardness to aragonite. There is usually little or no reactionbetween cold HCl and dolomite. Dolomiterock is quite widespread andcommonly considered to be diagenetic. Siderite

Siderite is iron carbonate (FeCO3) with the samestructure as calcite and it is difficult to distinguishbetween iron and calcium carbonates on mineralogicalgrounds. Siderite forms within sediments as an earlydiagenetic mineral (Nichols, 2009) (Table 2).

Table 2: Common carbonate minerals

3.0 Carbonate Precipitation

Precipitation, preservation and alteration of the carbonaterocks are strongly influenced by their mineralogy. Threeminerals appear in significant amounts: aragonite, calcite and dolomite. Calcite, aragonite and dolomite differ considerably in solubility and the differences are sedimentologically very important.In sea water (and also in many pore waters) theranking in terms of solubility is aragonite > calcite > dolomite.

Nearly all marine surface waters aresupersaturated with respect to calcite and dolomite but theappropriate precipitation reactions are blocked in variousways. The abiotic reactions that ultimately do occur, suchas the formation of fibrous cements inmarine environments,produce aragonite or magnesian calcite rather than the thermodynamicallyexpected minerals calcite and dolomite(Schlager, 2005).Precipitation of solid matter from the dissolved load ofthe sea occurs either abiotically, governed by inorganic thermodynamicsand reaction kinetics, or biotically as a consequenceof the metabolism of plants and animals. Marine carbonate precipitationproceeds along abiotic and biotic pathways and this makesit particularly diverse and complex.

 Abiotic (or quasi-abiotic): Precipitates where biotic effectsare negligible.  Biotically induced: Precipitates where the organism setsthe precipitation process in motion but organic influenceon its course is marginal or absent. The reactiontakes place outside the cell and the product is very similar,often indistinguishable, from abiotic precipitates.  Biotically controlled: Precipitates where the organism determines location, beginning and end of the processand commonly also composition and crystallography ofthe mineral. All skeletal carbonate falls in this category (Fig. 2).

Fig. 2: Carbonate precipitation (Schlager, 2000) 3.1 Abiotic marine carbonate precipitation Organisms and organic matter are so common in depositional environments and have so many ways of influencing carbonate precipitation that it is virtually impossible to demonstrate that a particular carbonate precipitate from a natural environment is totally abiotic. However, there are carbonate materials for which the organic influence, if present at all, is very subtle. Their texture and mineralogy can be reproduced abiotically in the laboratory and their natural occurrence is governed by first-order trends in ocean chemistry. The most conspicuous abiotic precipitate is cement formed in the pore space during the early stages of diagenesis when the deposit was still in the depositional environment. Burial cements are excluded from the abiotic carbonate factory because they are not derived from sea water but largely from remobilized sedimentary material. The case for abiotic origin is particularly strong for acicular aragonite cements. Acicular magnesian calcites may be biotically influenced (Morse and Mackenzie, 1990). Abiotic carbonate precipitation may be summarized as-

3.2 Biotically controlled precipitation Marine carbonate precipitation and deposition are closelyrelated to life in the ocean. Ecology, the study of the relationshipof organisms and their environment, provides a numberof very useful concepts for carbonate sedimentologists.Allinteractions in a given ecosystem constitute the food chain.Marine carbonate production nearly always dependson photosynthesis as a starting point.The organisms at the starting point of the food chain arecalled autotrophs (literally: self- feeders); organisms furtherdown the food chain depend on other organisms for foodand are called

heterotrophs.The rate of photosynthetic production, the primary productivity,in the marine environment depends on the lightintensity and the concentration of dissolved nutrients, suchas phosphorous, nitrogen or carbon. The majority of carbonate material in modern oceans isprecipitated as highly structured skeletons of organisms.Precipitation is primarily controlled by the biochemistryof the respective organisms (such as algae, foraminifera orcorals); the organisms, in turn, are influenced by the conditionsof the sea they live in, particularly light, temperatureand water chemistry (for instance the degree of carbonatesaturation of the sea water).

5.0 Classification of Carbonates Like any other classification, carbonate rocks also have several schemes of classification with each having its own importance. Most popular are classification of Folk (1959, 1962) and Dunham (1962). Main criteria for classification of carbonate sediments are-  Mineralogy  Chemical composition  Types of carbonate grains  Depositional texture  Textural maturity  Diagenetic factors  Origin (Organic/ Inorganic)  Energy level  Depositional environment  Folk’s classification (Folk, 1962) This classification is based on the principles of comparison of carbonate rocks with sandstone and shale in the purview of their textural characteristics controlled bydepositional energy. Their textures are largely controlled by wave energy at the site of deposition (Fig. 6). Major constituents of Folks’s classification are-  Allochems: These are the grains or particles chemically derived but have underone slight transportation within the basin. The main types of allochems are intraclasts, oolites, pellets/ pelloids and skeletal grains or fossils.

 Orthochems: These are the particles which have been chemically precipitated and usually remain in-situ. These are- micrite/ microcrystalline calcite and sparite/ sparry calcite cement.

Fig. 6: Folk’s textural classification of carbonates (Folk, 1962)

Dunham’s classification (Dunham, 1962): The Dunham Classification is the most widelyused scheme for the description of limestone and primarycriterion used in this classification scheme is thetexture, which is described in terms of the proportionof carbonate mud present and the framework of therock. The first stage in using the Dunhamclassification is to determine whether the fabric ismatrix- or clast-supported. Matrix- supported limestoneis divided into carbonate mudstone (less than10% clasts) and wackstone (with more than 10%clasts). If the limestone is clast-supported it is termed apack stone if there is mud present or a grain stone ifthere is little or no matrix. A bound stone has anorganic framework such as a coral colony (Fig. 7).

Fig. 7: Classification of limestone (Dunham, 1962)

6.0 Depositional Environments Limestones are predominantly form in shallow marine environment but they nay also form in non-marine, lacustrine and pelagic environments. Limestones make a major part of stratigraphic record across the globe. The environments in which carbonates are formed require certain restricted conditions that are found only on shallow shelves between 300N and 300S latitudes as carbonates are produced by environmentally sensitive organisms. Limestones in the modern ocean are deposited in several kinds of platform/shelf environments,which include rimmed platforms, open shelves, ramps, isolated platforms, and epeiric platforms (Fig. 8). Peritidal environments are restricted to the clear, shallow subtropical to tropical marine shelves which occur in stable tectonic settings with little relief. Shelf carbonates require warm, clear, shallow, well oxygenated, normal marine conditions which typically exist only on continental shelves or epeiric seas in low latitudes with no significant clastic input (Fig. 9). Reef and build-ups form at the edge of carbonate banks where upwelling from deeper waters brings up nutrients. They are extremely limited by depth, temperature, salinity and nutrients.

Fig. 8: Carbonate ramp setting

Fig. 9: Schematic representation of principal kinds of marine platform/shelf carbonatesenvironments (Boggs, 2006)

7.0 Diagensis The diagenesis of carbonate sediments encompassesall the processes which affect the sediments afterdeposition until the realms of incipient metamorphismat elevated temperatures and pressures. Diagenesisincludes obvious processes such as cementation toproduce limestones and dissolution to form cave systemsbut it also includes more subtle processes such asthe development of microporosity and changes intrace element and isotopic signatures. Diageneticchanges can begin on the seafloor, as the grains arestill being washed around or as a reef are still growing,or it may hold off until burial when overburdenpressure has increased or pore-fluid chemistry haschanged so that reactions are then induced within thesediments.Most modern and many ancient marine carbonatesediments originally consist of a mixture of aragonite,high-Mg calcite and low-Mg calcite, and in the Recentthe first two minerals are commonly precipitated ascements from marine pore-fluids during early diagenesis.Unlessdolomitized, the majority of ancient limestones arecomposed entirely of low-Mg calcite. Studies of limestonediagenesis thus often involve trying to identifythe original mineralogy of the various cements, theirsignificance in terms of pore-fluid chemistry and diageneticenvironment and the relative timing of precipitationand any alteration. The manner in whichthe grains themselves are preserved is also mostimportant(Tucker et al., 1990). Diagenesis includes six major processes: cementation, microbial micritization, neomorphism, dissolution, compaction (including pressure dissolution)and dolomitization.The patterns of diagenesis mayvary greatly from one limestone formation to another,and there are frequently variations both laterally andvertically within one limestone sequence. Major controlson the diagenesis are the composition andmineralogy of the sediment, the pore-fluid chemistryand flow rates, geological history of the sediment interms of burial/uplift/sea-level changes, influx of different pore-fluids and prevailing climate.During diagenesis carbonate sediments may gainor lose porosity. With increasing depth of burial,there is generally a decrease in porosity butthere are late processes of dissolution and fracturewhich can restore higher porosity values. Carbonate diagenesis operates in three principalenvironments: the marine, near-surface meteoric andburial environments, and there are features of thecement fabrics and other textures which are diagnosticof a particular diagenetic environment (Fig. 10).

Cementation. The precipitation of cements in carbonatesediments is a major diagenetic process and takesplace when pore-fluids are supersaturated with respectto the cement phase and there are no kinetic factorsinhibiting the precipitation. Petrographic and geochemicalstudies of these cements enable deductionsto be made of the environment and conditions of cementation. Organic geochemical influences are importantin some instances. Aragonite, high-Mg calcite,low-Mg calcite

and dolomite are the common carbonatecements in limestones and they comprise a rangeof morphologies. Less commonly, ankerite, siderite,kaolinite, quartz, anhydrite, gypsum and halite arethe cements.

Microbial micritization. This is a process wherebybioclasts are altered while on the seafloor or justbelow by endolithic algae, fungi and bacteria. Theskeletal grains are bored around the ma rgins and theholes filled with fine-grained sediment or cement.Micritic envelopes areproduced in this way and if the activity of the endolithicmicrobes is intense, completely micritized grains arethe result. The original skeletal nature of such grainsis often difficult to determine. The usually ratherirregular shape of these micritized grains distinguishesthem from the micritic faecal pellets(Tucker et al., 1990).

Fig. 10: Diagenetic environment Neomorphism:Processes of replacement and recrystallizationwhere there may have been a change ofmineralogy. Recrystallization, strictly, refers tochanges in crystal size without any change of mineralogy.Neomorphic processes take place in the presence of waterthrough dissolution- reprecipitation. Most neomorphism in limestones is of theaggrading type that is leading to a general increase in crystal size and this occurs chiefly in fine-grainedlimestones, resulting in microsparitic patches, lenses,laminae and beds.

Dissolution. Carbonate sediments and cements andpreviously lithified limestones may undergo dissolutionon a small or large scale when pore-fluids areundersaturated with respect to the carbonate mineralogy.Individual grains may be dissolved out, especiallyif they are of a metastable mineralogy likearagonite. Dissolution is particularly importantin near-surface meteoric

environments. With limestones, vugs, potholes,caverns and cave systems may develop as a result ofdissolution through karstification. This may happensoon after deposition, or much later when limestone isuplifted.

Compaction. When carbonate sediments are buriedunder an increasing overburden, then if they are notalready cemented, grain fracture takes place and porosityis lowered by a closer packing. This is mechanical compaction.Eventually grains begin to dissolve atpoint contacts to produce sutured and concavo-convexcontacts. This chemical compaction also takesplace in previously lithified limestones to generatestylolites and dissolution seams, the latter sometimesreferred to as flasers(Tucker et al., 1990) (Fig. 11).

Fig. 11: Summary of main diagenetic processes and porosity response

8.0 Conclusions Carbonates are the most abundant non-terrigenous rocks constituting about 10% of earth’s sedimentary shell. In contrast to terrigenous rocks, they form chemically and biochemically. Any - process which removes CO2 from the normal water tends to change to bicarbonate (HCO3 ) to 2- carbonate (CO3 ) ions encourages lime precipitation. To understand reservoir properties of carbonates it is important to know small scale variations in lithology, depositional facies and diagenetic environments responsible for heterogeneity of carbonates.Carbonates are distributed throughout the geological periods from Precambrian to Recent but with peak deposition during Late Devonian, Late Carboniferous, Early Permian, Late Jurassic, Middle Cretaceous and Miocene. Carbonates are important reservoirs for hydrocarbons but also very important to understand depositional environment. They also host important ore bearing minerals. Important carbonate minerals are calcite, aragonite, dolomite, siderite and ankerite. They vary in crystal system, shape and specific gravity along with mineralogy. Chemically calcite and aragonite have same composition. Carbonates form in shallow, warm, clean marine water which

commonly exists between 300N and 300S latitudes which promotes photosynthesis. Carbonate deposition require minimum or no supply of clastic sediments. Carbonates have grains, matrix and cement as main components. Like other rocks, carbonates are also classified based on wide variety of factors. Folk and Dunham’s classifications are most common and widely used. Diagenesis is more effective in carbonates than in siliciclastic rocks. Majority of carbonate reservoirs comprise secondary porosity resulted out of diagenetic processes. Porosity is substantially reduced due to compaction and cementation but is enhanced by dissolution, shrinkage and fracturing. In the carbonates primary porosity is framework porosity, intra-particles porosity, inter-particle porosity, shelter posrosity and fenestral porosity. Secondary porosity is classified as intercrystalline porosity, moldic porosity, vugs and channel porosity and fracture- breccia porosity.