CARBONATE RESERVOIR ROCK PROPERTIES
Fundamental rock properties include texture, composition, sedimentary structures, taxonomic diversity, and depositional morphology. The last two properties are not commonly listed as “fundamental rock properties”in most texts but they are important attributes of sedimentary deposits that must be included in thorough reservoir studies. Fundamental rock properties provide the basis for defining lithofacies, or lithogenetic units that make up depositional reservoirs. Diagenetic and fractured reservoirs are simply altered versions of the original depositional version. The most reliable method for identifying these fundamental properties in carbonates is direct observation of cores or cuttings. Cores provide enough sample volume to determine sedimentary textures, grain types, sedimentary structures, and biota. Cuttings usually provide enough volume to determine mineralogy, grain types, and estimates of texture. Logs are not very helpful in identifying fundamental rock properties in carbonates. Facies types can be identified in siliciclastic sandstones by using the shape of the gamma ray and resistivity or, with older logs, the SP – resistivity log traces. When the paired traces outline a bell, a funnel, or a cylinder, the corresponding sandstone facies are assumed to be channel - fill, deltaic, or reworked sheet sands, respectively. Other “typecurves” are assumed to be indicators of other of sand – shale depositional successions. The underlying assumption is that the gamma ray, SP, and resistivity logs are sensitive to vertical changes in grain size. In fact, that assumption is false. The logs are not sensitive to grain size. The gamma ray tool measures natural radioactivity that issues from the K, Th, and U found in clay minerals that are commonly incorporated in shales and mudrocks. The tool does not measure grain size. In fact, “ hot limes ” and “ hot dolomites ” are commonly found in carbonate reservoirs where particle size has nothing to do with the presence of natural radioactivity. The SP and resistivity tools likewise measure electrical properties of the rock – fluid system and shales tend to have less deflection from the log baseline than coarser grained sections that have bigger fluid - filled pores. Mineralogical composition is used to classify sandstones but not carbonates. Carbonate rock classification is based on grain type and depositional texture. Mineralogy may be strongly correlated with porosity in carbonates but it has much less influence on sandstone porosity. Sedimentary structures and biota can only be determined with complete certainty by observing borehole cores. Sedimentary structures provide clues to the hydrodynamics and directions of flow in ancient environments in both terrigenous sandstones and carbonates. In some cases, image logs and sensitive dip- meters can detect larger sedimentary structures such as large - scale cross-bedding in dunes. Fossil content is arguably more important for interpreting depositional environment in carbonates than in terrigenous sandstones probably because mostcarbonates form in marine environments where fossil assemblages can reveal subtle differences in depositional settings. Diverse assemblages of fossils indicate favorable environment for life. Low diversity indicates a stress environment such as a hyper - or hyposaline lagoon, low oxygen content, or some other limiting factor on life. Low diversity is rarely associated with grain - supported or reef rocks; therefore low diversitycan be a negative indicator for depositional porosity in reservoir rocks.
The fundamental rock propertiesare used to classify both rocks and porosity, and how fundamental rock propertiesare related to reservoir properties.
FUNDAMENTAL PROPERTIESof CARBONATE RESERVOIR Fundamental properties of carbonate rocks include texture, fabric, grain type, mineralogicalcomposition, and sedimentary structures. Note that texture and fabric arenot interchangeable terms. Texture is defined as the size, shape, and arrangement ofthe grains in a sedimentary rock (Pettijohn, 1975). Among carbonate sedimentologists,texture is sometimes thought of in the context of depositional texture, whichforms the basis for several carbonate rock classification systems. Fabricrefers to thespatial arrangement and orientation of the grains in sedimentary rocks. It can alsorefer to the array geometry or mosaic pattern of crystals in crystalline carbonatesand the growth form (macroscale) and skeletal microstructure (microscale) of reeforganisms. Mineralogical compositionrefers to original mineralogy. Original mineralogicalcomposition has great significance in the study of carbonate diagenesis andit provides important clues about the chemical evolution of the earth. It is not,however, a reliable clue to the origin and distribution of reservoir flow units becausecarbonates in a wide variety of depositional settings may consist of calcite, aragonite,or dolomite, individually or in mixtures. It is more practical for the reservoir geoscientistto substitute constituent grain type, such as skeletal grains, peloids, clasts,or ooids, among others, for composition.
Sedimentary structuresare preserved bedformscreated by fluid processes acting on the sediment interface, by desiccation,slope failure, thixotropy, compaction, fluid expulsion, and bioturbation by burrowing and boring organisms.
1. Texture There are many textural terms in the literature on sedimentary rocks, but mostgeologists today describe grain sizes according to the Wentworth (1922) scale inmillimeters, or in “ phi units, ” which are logarithmic transformations to the base 2of the size (in millimeters). It is rarely possible to disaggregate lithified limestonesinto component grains; consequently, direct size measurements by sieve, pipette, or hydrometer are limited to unconsolidated sediments. Estimates of grain size can bemade from thin sections of lithified carbonates, although the method requiresstatistical manipulation of grain size measurements to compensate for the factthat two - dimensional microscope measurements do not provide the true three - dimensional grain size. Tucker (1988) and Tucker and Wright (1990) discuss theproblem of determining grain sizes from thin section measurements in moredetail. The Wentworth scale (Figure ) classifies all grains with average diametersgreater than 2 mm as gravel , those with average diameters between 2 mmand 116 mm (62 μ m) as sand , and those finer than 62 μ m as mud . In this context, sanddenotes texture rather than composition. Other terms for gravel, sand, and mudinclude the Greek derivatives psephite, psammite, and pelite, but they are rarelyused in modern literature. The Latin terms rudite, arenite, and lutite appear in thecomprehensive but unwieldy sedimentary rock classification scheme of Grabau(1960). The terms appear in modern literature as calcirudite, calcarenite, and calcilutite, indicating carbonate gravel, sand, and mud, respectively.
Embry and Klovan(1971) blended rudite with Dunham ’ s (1962) carbonate rock classification terminologyto create rudstone in their classification of reef carbonates. Lithified lime mudthat exhibits a mosaic of calcite crystals 1 – 4 μ m in diameter became known as micrite , a contraction of microcrystalline and calcite , coined by Folk (1959) . Someworkers now classify all carbonate mud, regardless of its size and mineralogicalcomposition, as micrite, even though that is inconsistent with the original definition. Much of this “micrite” is actually calcisiltite , or silt - sized (62 μ m to 3.90 μ m) sediment.Note that chalk is a special rock type that is not generally classified as micriteor mud. True chalk consists of cocolith skeletal fragments, usually in a grain -supported fabric. Coccolithophorids are flagellated yellow - green algae that produce a spheroidal mass of platelets that become disarticulated after death and rain downto the sea floor as disk - shaped particles 2 – 20 μ m in diameter (Milliman, 1 974). Electronmicrographs of chalk show grain - supported depositional textures without amatrix of aragonite or calcite crystals finer than the cocoliths; therefore chalk is notstrictly a mud or micrite in the sense of the detrital micrites described earlier. Of course, there are “gray” areas. Calcisiltites (lime muds) may contain some cocoliths,but they are not proper chalks. Grain size is not generally as useful for interpreting ancient hydrologic regimesin carbonate depositional environments as it neither is with terrigenous sandstones nor isgrain size consistently related to carbonate reservoir porosity or permeability.
Carbonate grain size terminology Grains > 2mm ( > sand grade) CALCIRUDITES Grains 2 - 0.063mm (sand grade) CALCARENITES (Calcareous sandstones) Grains < 0.063mm (mud grade) CALCILUTITES (Calcareous mudstones or micrite)
Carbonatesconsist mainly of biogenic particles that owe their size and shape to skeletalgrowth rather than to a history of mechanical transport, deposition, and arrangement. Most carbonate grains originate in the marine environment where waves andcurrents fragment, winnow, and sort sediment, primarily along strand plains andon slope changes (usually associated with bathymetric highs) that occur above
2. Fabric Depositional, diagenetic, or biogenic processes create carbonate rock fabrics. Tectonicprocesses such as fracturing and cataclasis are not part of the depositional andlithification processes but may impart a definite pattern and orientation to reservoirpermeability. Fractured reservoirs are discussed later. Depositional fabric is the spatial orientation and alignment ofgrains in a detrital rock. Elongate grains can be aligned and oriented by paleocurrents. Flat pebbles in conglomerates and breccias may be imbricated by unidirectionalcurrent flow. These fabrics affect reservoir porosity and can impart directionalpermeability, ultimately affecting reservoir performance characteristics. Elongateskeletal fragments such as echinoid spines, crinoid columnals, spicules, some foraminifera, and elongate bivalve and high - spired gastropod shells are common in carbonate reservoirs. Presence or absence of depositional fabric is easily determinedwith core samples; however, determination of directional azimuth requires orientedcores. In some cases, dipmeter logs and high - resolution, borehole scanning andimaging devices may detect oriented features at the scale of individual beds orlaminae (Grace and Pirie, 1986). Diagenetic fabrics include patterns of crystal growth formed duringcementation, recrystallization, or replacement of carbonate sediments and fabricsformed by dissolution. Dissolution fabrics include a wide range of features such asmolds, vugs, caverns, karst features, and soils. Mold and vug characteristics may bepredictable if dissolution is fabric - or facies - selective; however, caverns, karst features,and soils may be more closely associated with paleotopography, paleoaquifers, or unconformities than with depositional rock properties. Without such depositionalattributes, dissolution pore characteristics are harder to predict. Intercrystallineporosity in dolomites and some microcrystalline calcites are fundamental propertiesbut they are diagenetic in origin. The size, shape, orientation, and crystal “ packing ”(disposition of the crystal faces with respect to each other) create an internal fabricthat greatly affects reservoir connectivity because they determine the size, shape, and distribution of pores and connecting pore throats. Biogenic fabrics are described in connection with carbonate buildups, or reefs,and with the internal microstructure of skeletal grains. A classification of reef rockswas conceived to cope with variability in reservoir characteristics within a singlereef complex (Embry and Klovan, 1971). They described three end - member biogenicfabrics, including (1) skeletal frameworks in which interframe spaces are filledwith detrital sediments, (2) skeletal elements such as branches or leaves that actedas “ baffles ” that were subsequently buried in the sediment they helped to trap, and(3) closely bound fabrics generated by encrusting organisms. The skeletal microstructureof many organisms is porous and may provide intraskeletal porosity, evenin non-reef deposits. The pores within sponge, coral, bryozoan, stromatoporoid, orrudist skeletons, for example, are intraparticle pores, although the individual skeletons are part of larger reef structures. All three fabric categories are closely relatedto reservoir properties because fabric influences pore to pore throat geometry andmay influence directional permeability. An example of combined biogenic anddetrital fabric is illustrated in a Pleistocene coral framestone reef with detrital interbeds.
3. Composition Composition of carbonate rocks usually refers to constituent grain type rather thanmineral content, because carbonates may be monomineralic and the mineral contentof polymineralic carbonates is not generally indicative of depositional environment. Carbonate grains are classified as skeletal and nonskeletal. Extensive, illustrated discussionsof constituents commonly found in carbonates of different geological agesare found in Bathurst (1975), Milliman (1974), Purser (1980), Scoffin (1987), andTucker and Wright (1990) . Skeletal constituents include whole and fragmentedremains of calcareous plants and animals such as mollusks, corals, calcified algae, brachiopods,arthropods, and echinoderms, among many others. Nonskeletal grainsinclude ooids, pisoids, peloids, and clasts. Ooids and pisoids are spheroidalgrains that exhibit concentric microlaminae of calcite or aragonite around anucleus. The marine variety is formed by chemical processes in agitated, shallowwater, usually less than 2 m deep (Tucker and Wright, 1990). Clasts areparticles produced by detrition (mechanical wear); they include resedimented fragmentsof contemporaneous or older rock known as intraclasts and lithoclasts, respectively,following Folk (1959).
Clasts indicate erosion and resedimentation of lithifiedor partly lithified carbonates, some of which may have been weakened by bioerosion(rock boring and grinding by specialized organisms) or by weathering. Peloidis an all - inclusive term coined by McKee and Gutschick (1969) to includerounded, aggregate grains of microcrystalline carbonate. Peloids are produced bychemical, biogenic, and diagenetic processes and typically form in shallow, warm, agitated, and carbonate-saturated waters such as those Aswan. Pellets differ in that true pellets are compacted bits offecal matter that have distinctive shapes or internal structures (Figure ). Pelletscan be useful in determining the environment of deposition (Moore, 1939). Peloidsthat were probably formed as fecal pellets are prominent constituents of Wilson’s(1975) “standard microfacies” in the “ restricted platform ” environment.
4. Sedimentary Structures Sedimentary structures are useful aids for interpreting ancient depositional environments. They may affect reservoir characteristics because their internal fabrics areusually oriented and there may be regular patterns of grain size change within them. Extensive discussions and illustrations of sedimentary structures can be found inAllen (1985), Purser (1980), Reading (1996), Reineck and Singh (1973), and Tuckerand Wright (1990).
1-Structures formed by deposition: Ordinary bedding planes with variations due to surface irregularities, or diagenesis.
2- Structures formed by biological growth patterns: Constructed voids, skeletal growth fabrics, and patterns of organic lamination (e.g., algal laminae); includesStromatactiscavities.
-Stromatactis A series of elongated cavities, with curved or irregular tops and flat bases, filled with calcite cements. Stromatactis cavities were originally believed to be of organic origin, but currently they are thought to result either from the dewatering of lime muds or from the development of cavities beneath local cemented crusts on the sea floor.
-Current-Generated Structures. Many shells of organisms have curved outlines in cross-section (brachipods, pelecypods, ostracods, and trilobites, especially), when the organism dies it may settle to the bottom with the outline being concave downward, and later become filled with carbonate mud. When such features occur they can be used as top/bottom indicators. - Lamination. The most common type of lamination in carbonate rocks is produced by organisms, in particular blue-green algae that grow in the tidal environment. These organisms grow as filaments and produce mats by trapping and binding microcrystalline carbonates, as incoming tides sweep over the sand. This leads to the formation of laminated layers that consist of layers of organic tissue interbedded with mud. In ancient limestones, the organic matter has usually been removed as a result of decay, leaving cavities in the rock separated by layers of material that was once mud. These cavities are called fenestrae. Another type of lamination occurs as bulbous structures, termed Stomatolites. These are produced in a similar fashion, i.e. by filamentous blue-green algae, but represent mounds rather than mats.
3- Structures formed by Compaction: Stylolites, diagenetic enhancement of bedding irregularities, and closure of intergranular pores Stylolites. Stylolites are irregular surfaces that result from pressure solution of large amounts of carbonate. In cross-section they have a saw tooth appearance with the stylolites themselves being made of insoluble residues or insoluble organic material. Some studies have suggested that the stylolites represent anywhere from 25% to as much as 90% of missing rock that has been dissolved and carried away by dissolution.
Varieties ofcarbonate rocks: • Coquina: a mechanically sorted and composed of loosely aggregated shells and shell fragments. • Chalk: It is a soft, white, porous, a form of limestone forms under relatively deep marine conditions from the gradual accumulation of minute calcite plates. Chalk is composed mostly of calcium carbonate with minor amounts of silt and clay. It is common to find Chert nodules embedded in chalk. Chalk can also refer to other compounds including magnesium silicate and calcium sulfate. •Dolomite: composed of calcium magnesium carbonate CaMg(CO3)2 •Marl: It is loosely consolidated mixture of siliciclastic clay and calcium carbonate, formed from porous mass of shells & shell fragments accumulate on the bottom of fresh water lakes. •Travertine: Travertine is a terrestrial sedimentary rock, formed by the precipitation of carbonate minerals from solution in ground and surface waters. Travertine forms the stalactites of limestone caves.A limestone that forms by evaporative precipitation, often in a cave, to produce formations such as stalactites, stalagmites and flowstone.
Fossiliferous Limestone: A limestone that contains obvious and abundant fossils. These are normally shell and skeletal fossils of the organisms that produced the limestone.
Tufa Tufa forms where a natural spring flows into Lake. Precipitation of calcium carbonate, and any other ions will occur instantaneously around the spring vent. This leads the development of tufa towers or bulbous cauliflower-shaped structures that are relatively porous when inspected closely.
Reefs
Reefs are sediment systems built entirely from the organisms that call it a home. It is a wave resistant framework. Modern reefs primarily exist in oligotrophic environments and this rival the rainforests for biodiversity. Reefs, which form at the edges of carbonate banks, can be excellent oil traps.
The architects of reefs (framework builders) include scleractinian coral, coralline algae, bryozoans and sponges, but in the past even microbial mats could built up reefs. However, framework builders are generally only 10% of the total volume of the reef, the remainder is composed of skeletal fragments, micrite, breccia and cements, which fill in the interstitial spaces of the reef framework.
Parts of the reef
Back-reef (lagoon) - low energy, lime muds; bordered by tidal flat on landward side
Reef - high energy, "boundstone"
Fore-reef (deep water) - turbidites, breccias, grading seaward into organic-rich lime mud
Corals are tiny marine animals (polyps) which live in small cone-like cells, commonly in warm, tropical waters. The animals have tentacles to assist feeding, and may seal the end of their cells with an operculum (lid). They often live in colonies, behaving either independently as individuals or with a degree of specialization of function so that the whole colony operates, to some extent, as an organism. Their skeletons often accumulate in vast quantities, sometimes as reefs, which may become consolidated as various types of limestone. There are many hundreds of different living species-700 alone in the Indo-Pacific region, and similar numbers of extinct species. Two extinct types of corals which are frequently preserved in limestones are the rugose and the tabulate corals, both of which arose in the Ordovician Period (434 to 490 million years ago) and became extinct at the end of the Permian Period (251 million years ago).
Thus largely due to mass extinction, the types of framework builders in reefs have changed through time.
Global Distribution of Reefs