CARBONATE RESERVOIR ROCK PROPERTIES
Transcript of CARBONATE RESERVOIR ROCK PROPERTIES
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