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
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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