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