Ahr W1 H. Geologyof Carbonate Reservoirs

Ahr W1 H. Geologyof Carbonate Reservoirs

(Parte 5 de 8)

FUNDAMENTAL ROCK PROPERTIES 17

so important

fair - weather wave base. However, carbonate grains are produced in a wide variety of environments; consequently, it is risky to use grain size and shape alone as indicators of the hydrologic regime. It is better to consider grain size, sorting, grain shape, amount and type of grain fragmentation, and mechanical durability of the grains as clues for interpreting depositional environments. Some grains are produced by bioerosion — boring, rasping, grinding, and digesting carbonate constituents by organisms. The biological reworking makes grain size and shape virtually useless as indicators of the hydrologic regime. Perhaps the most effective villain in altering carbonate grain size and shape is diagenesis. Micrite can be produced from sand and gravel, grains can be enlarged by cement overgrowths, and shape can be changed by cementation or dissolution. Fortunately, diagenetic changes are not diffi cult to identify in thin sections. Sorting and grain size (Figure 2.2 ) are textural attributes that can be useful in studying carbonate rocks because they infl uence depositional porosity and permeability. Porosity is independent of grain size where grains are ideal spheres, but permeability varies with particle size because small grains have small intergranular pores with small pore throats. Sorting and grain packing are also strongly related to permeability because sorting and packing infl uence the geometrical relationship between pores and pore throats. A high correlation exists between permeability and pore throat dimensions, as we will see later in our discussions on capillary pressure, permeability, and reservoir quality, but pore geometry alone is not strongly related to permeability. It is the pore – pore throat relationship that is

Mechanical abrasion, along with hydraulic size and shape sorting, are important processes in beach, dune, and shallow, slope - break environments where mud - free

Figure 2.2A plot showing the relationships between grain size, sorting, and porosity in

unconsolidated sands. Based on data in Beard and Weyl (1973) and adapted from unpublished notes with permission from R. M. Sneider (1988) . Note that porosity does not vary with grain size but does vary with sorting. Permeability varies with both grain size and sorting.

5.7 Coarse Medium Fine Very Fine Silt

Very Poor

Poor

Moderate

Well Very Well

Extremely Well

Median Grain Size – m

Grain Size

Sor ting

Sor ting – S

18CARBONATE RESERVOIR ROCK PROPERTIES deposits are produced. The removal of mud by winnowing is important because the leading carbonate rock classifi cations are based on the presence or absence of mud. Rocks with high mud content usually represent sediment “ sinks, ” or areas where water movement has been slow and mud has settled out of suspension. If fractures, dissolution diagenesis, or alteration to microporous crystalline fabrics are absent, muddy carbonates are not good reservoir rocks. They may be good source rocks instead if they contain enough sapropelic (lipid - rich) organic matter. Carbonate sands and gravels with little or no mud represent either effective winnowing or a lack of mud production. Well - sorted and mud - free carbonate rocks have high depositional porosity and permeability.

2.2.2 Fabric

Depositional, diagenetic, or biogenic processes create carbonate rock fabrics. Tectonic processes such as fracturing and cataclasis are not part of the depositional and lithifi cation processes but may impart a defi nite pattern and orientation to reservoir permeability. Fractured reservoirs are discussed in Chapter 7 .

laminae (Grace and Pirie, 1986 )

Depositional fabric (Figure 2.3 a) is the spatial orientation and alignment of grains in a detrital rock. Elongate grains can be aligned and oriented by paleocurrents. Flat pebbles in conglomerates and breccias may be imbricated by unidirectional current fl ow. These fabrics affect reservoir porosity and can impart directional permeability, ultimately affecting reservoir performance characteristics. Elongate skeletal 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 determined with core samples; however, determination of directional azimuth requires oriented cores. In some cases, dipmeter logs and high - resolution, borehole scanning and imaging devices may detect oriented features at the scale of individual beds or

Diagenetic fabrics (Figure 2.3 b) include patterns of crystal growth formed during cementation, recrystallization, or replacement of carbonate sediments and fabrics formed by dissolution. Dissolution fabrics include a wide range of features such as molds, vugs, caverns, karst features, and soils. Mold and vug characteristics may be predictable 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 depositional attributes, dissolution pore characteristics are harder to predict. Intercrystalline porosity in dolomites and some microcrystalline calcites are fundamental properties but 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 fabric that 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 classifi cation of reef rocks was conceived to cope with variability in reservoir characteristics within a single reef complex (Embry and Klovan, 1971 ). They described three end - member biogenic fabrics, including (1) skeletal frameworks in which interframe spaces are fi lled

FUNDAMENTAL ROCK PROPERTIES 19

with detrital sediments, (2) skeletal elements such as branches or leaves that acted as “ baffl es ” that were subsequently buried in the sediment they helped to trap, and (3) closely bound fabrics generated by encrusting organisms. The skeletal microstructure of many organisms is porous and may provide intraskeletal porosity, even in nonreef deposits. The pores within sponge, coral, bryozoan, stromatoporoid, or rudist skeletons, for example, are intraparticle pores, although the individual skeletons are part of larger reef structures. All three fabric categories are closely related to reservoir properties because fabric infl uences pore to pore throat geometry and may infl uence directional permeability. An example of combined biogenic and detrital fabric is illustrated in a Pleistocene coral framestone reef with detrital interbeds (Figure 2.3 c).

Figure 2.3(a) Depositional fabric in detrital rocks. Grain orientation and alignment pro-

duced by currents at time of deposition. The larger grains at the top of the fi gure are imbricated and those at the bottom of the fi gure are simply oriented with long axes parallel to the direction of fl ow. Permeability is highest in the direction of grain alignment. (b) Diagenetic fabric. Complete replacement of limestone by dolomite creates a diagenetic fabric totally unrelated to depositional rock properties. In this case the dolomite rhombohedra occur in an “ open ” fabric with a great amount of intercrystalline porosity. (c) Biogenic (skeletal growth) fabric. The great variety of internal growth fabrics created by reef - building organisms creates depositional fabrics dramatically different from those in detrital (particulate) rocks. The photo illustrates a Pleistocene patch reef from Windley Key, Florida. Porosity and permeability in the coral framestone are infl uenced by biologically constructed, skeletal structures, not by granular or crystalline fabrics.

Current A

6 in

20CARBONATE RESERVOIR ROCK PROPERTIES

2.2.3 Composition

(1975) “ standard microfacies 8 ” in the “ restricted platform ” environment

Composition of carbonate rocks usually refers to constituent grain type rather than mineral content, because carbonates may be monomineralic and the mineral content of polymineralic carbonates is not generally indicative of depositional environment. Carbonate grains are classifi ed as skeletal and nonskeletal . Extensive, illustrated discussions of constituents commonly found in carbonates of different geological ages are found in Bathurst (1975) , Milliman (1974) , Purser (1980) , Scoffi n (1987) , and Tucker and Wright (1990) . Skeletal constituents include whole and fragmented remains of calcareous plants and animals such as mollusks, corals, calcifi ed algae, brachiopods, arthropods, and echinoderms, among many others. Nonskeletal grains include ooids, pisoids, peloids, and clasts. Ooids and pisoids (Figure 2.4 a) are spheroidal grains that exhibit concentric microlaminae of calcite or aragonite around a nucleus. The marine variety is formed by chemical processes in agitated, shallow water, usually less than 2 m deep (Tucker and Wright, 1990 ). Clasts (Figure 2.4 b) are particles produced by detrition (mechanical wear); they include resedimented fragments of contemporaneous or older rock known as intraclasts and lithoclasts, respectively, following Folk (1959) . Clasts indicate erosion and resedimentation of lithifi ed or partly lithifi ed carbonates, some of which may have been weakened by bioerosion (rock boring and grinding by specialized organisms) or by weathering. Peloid (Figure 2.4 c) is an all - inclusive term coined by McKee and Gutschick (1969) to include rounded, aggregate grains of microcrystalline carbonate. Peloids are produced by chemical, biogenic, and diagenetic processes and are important constituents of shallow marine platform sediments. Pellets differ in that true pellets are compacted bits of fecal matter that have distinctive shapes or internal structures (Figure 2.4 d). Pellets can be useful in determining the environment of deposition (Moore, 1939 ). Peloids that were probably formed as fecal pellets are prominent constituents of Wilson ’ s

2.2.4Sedimentary Structures

and Wright (1990)

Sedimentary structures are useful aids for interpreting ancient depositional environments. They may affect reservoir characteristics because their internal fabrics are usually oriented and there may be regular patterns of grain size change within them. A complete discussion of sedimentary structures and their hydrodynamic signifi - cance is beyond the scope of this book. Instead, representative categories of sedimentary structures are grouped in Table 2.1 according to origin. Brief descriptions are included on the characteristics that distinguish the types of sedimentary structures, their environmental signifi cance, and their potential infl uence on reservoir performance. Some common sedimentary structures are illustrated in Figure 2.5 . Extensive discussions and illustrations of sedimentary structures can be found in Allen (1985) , Purser (1980) , Reading (1996) , Reineck and Singh (1973) , and Tucker

2.3CLASSIFICATION OF CARBONATE ROCKS

There are many classifi cation schemes for carbonate rocks. In 1904 Grabau devised one of the most comprehensive, but it is cumbersome and has never been popular

Figure 2.4 (a) Photomicrograph of a lime grainstone with ooids, intraclasts, and pisoids. Note the “ dogtooth spar ” isopachous rim cement on grain surfaces. Porosity in this rock is mainly intergranular but the grains have been altered and intragranular microporosity is also present. The horizontal width of the photo is 2.5 m. (b) Photomicrograph of an intraclastic (nonskeletal grains) conglomerate from the Cambrian of Central Texas. Note that the clasts are aligned in a fabric created by currents at the time of deposition. The width of the photo is 4 m. (c) Photomicrograph of an intraclastic, peloidal grainstone from the Cambrian of Central Texas. The width of the photo is 2.5 m. The most comon peloids are probably microbial in origin and, of those, most are found as cavity - fi llings in reefs and mounds. (d) Photomicrograph of ovoid, polychaete worm fecal pellets from the Pleistocene Campeche Calcilutite, Yucatan ramp, Mexico. The long axis of each pellet is about 2 m.

(Grabau, 1960 ). Popular, modern classifi cations for detrital carbonates were developed by Folk (1959, 1962) (Figure 2.6 ) and Dunham (1962) (Figure 2.7 ). Classifi cations for reef rocks were developed by Embry and Klovan (1971) (Figure 2.8 ) and Riding (2002) . A scheme to include depositional, diagenetic, and biological aspects of carbonates in one classifi cation system was proposed by Wright (1992) . There are two main purposes for classifi cation systems: (1) to make descriptions of

CLASSIFICATION OF CARBONATE ROCKS 21

TABLE 2.1 Physical Processes, Their Sedimentary Structures, and Infl uence on Reservoir Properties

Formative Processes

Sedimentary Structures

Descriptive Characteristics

Environmental Association

Infl uence on Reservoir Performance

Deposition only — Ordinary bedding

Ordinary bedding “ planes ” with variations due to surface irregularities, bioturbation, or diagenesis

May be even, wavy, or nodular; continuous or not over m - scale distances; beds parallel or not; thicknesses from millimeter to meter scale

Sediment “ sinks ” — areas of low - velocity fl uid motion; may be protected shallows, deeper zones below wave agitation, and areas free from current scour

May confi ne permeability to horizontal direction

Deposition only — Crossbedding

Commonly called “ spillover beds ” or “ simple cross stratifi cation ”

Curved, convex - upward bedding where lower boundaries of bed sets are nonerosional; scale varies from centimeter to multi - metersize; bed curvature diffi cult to detect in small

- diameter cores

(Parte 5 de 8)

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