CARBONATE RESERVOIR ROCK PROPERTIES Fundamental Rock

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

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