Page 34 - Geology of Carbonate Reservoirs

FUNDAMENTAL ROCK PROPERTIES 15

not interchangeable terms. Texture is defined as the size, shape, and arrangement of
the grains in a sedimentary rock (Pettijohn, 1975 ). Among carbonate sedimentolo-
gists, texture is sometimes thought of in the context of depositional texture , which

forms the basis for several carbonate rock classification systems. Fabric refers to the
spatial arrangement and orientation of the grains in sedimentary rocks. It can also
refer to the array geometry or mosaic pattern of crystals in crystalline carbonates
and the growth form (macroscale) and skeletal microstructure (microscale) of reef
organisms. Mineralogical composition refers to original mineralogy. Original miner-

alogical composition has great significance in the study of carbonate diagenesis and
it 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 because

carbonates 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 geo-
scientist to substitute constituent grain type, such as skeletal grains, peloids, clasts,
or ooids, among others, for composition. Sedimentary structures are preserved bed-

forms created by fluid processes acting on the sediment interface, by desiccation,
slope failure, thixotropy, compaction, fluid expulsion, and bioturbation by burrowing

and boring organisms. These definitive rock properties are discussed in more detail

in the following sections.
2.2.1 Texture

There are many textural terms in the literature on sedimentary rocks, but most
geologists today describe grain sizes according to the Wentworth (1922) scale in
millimeters, or in “ phi units, ” which are logarithmic transformations to the base 2
of the size (in millimeters). It is rarely possible to disaggregate lithifi ed limestones
into component grains; consequently, direct size measurements by sieve, pipette, or
hydrometer are limited to unconsolidated sediments. Estimates of grain size can be

made from thin sections of lithified carbonates, although the method requires
statistical manipulation of grain size measurements to compensate for the fact
that two - dimensional microscope measurements do not provide the true three -
dimensional grain size. Tucker (1988) and Tucker and Wright (1990) discuss the
problem of determining grain sizes from thin section measurements in more
detail.
The Wentworth scale (Figure 2.1 ) classifies all grains with average diameters

greater than 2 mm as gravel , those with average diameters between 2 mm
1

and mm (62 μ m) as sand , and those fi ner than 62 μ m as mud . In this context, sand
16
denotes texture rather than composition. Other terms for gravel, sand, and mud
include the Greek derivatives psephite, psammite, and pelite, but they are rarely
used in modern literature. The Latin terms rudite, arenite, and lutite appear in the
comprehensive but unwieldy sedimentary rock classification scheme of Grabau

(1960) . The terms appear in modern literature as calcirudite , calcarenite , and calci-
lutite , indicating carbonate gravel, sand, and mud, respectively. Embry and Klovan
(1971) blended rudite with Dunham ’ s (1962) carbonate rock classifi cation terminol-

ogy to create rudstone in their classification of reef carbonates. Lithified lime mud

that exhibits a mosaic of calcite crystals 1 – 4 μ m in diameter became known as

micrite , a contraction of micr ocrystalline and calc ite , coined by Folk (1959) . Some
workers now classify all carbonate mud, regardless of its size and mineralogical
composition, as micrite, even though that is inconsistent with the original defi nition.