Page 46 - Geology of Carbonate Reservoirs

CLASSIFICATION OF CARBONATE ROCKS 27

2.3.1 Classification of Detrital Carbonates

The Folk (1959, 1962) and Dunham (1962) classifications work well for detrital car-
bonates but they are not as useful to describe reef rocks or diagenetic textures and
fabrics. Folk and Dunham coined words to describe reef rocks. Folk chose biolithite
and Dunham chose boundstone , but those terms treat all reefs alike and as if the
entire reef mass were homogeneous. Porosity and permeability vary greatly in reefs
depending on the type of reef organisms present, the reef growth forms, the ratio
of skeletal framework to loose detritus, and the internal microstructure of the reef -
building organisms. Embry and Klovan (1971) and Riding (2002) devised reef clas-

sifications that include more detailed systems for describing the variability found in
reef reservoirs. Diagenetic properties are not included in the Folk and Dunham
schemes either, except that Dunham included the term crystalline carbonate for a
rock in which depositional texture is unrecognizable because it was obliterated by
diagenesis.

The Folk and Dunham classifications share a common theme. They are based on
the mud - to - grain ratios in carbonate rocks and on the packing arrangement of the
framework grains. These similarities exist because Dunham and Folk shared discus-
sions on carbonate classifications (Folk, 1962 ) and how the concept of textural

maturity used for terrigenous sandstones could be applied to carbonates. Textural
maturity in terrigenous sandstones refers to the amount of matrix (clay or mud)
that has been removed by winnowing and the extent to which sorting and rounding
are visible in framework grains. When applied to carbonates, rocks with high lime
mud content (more than 90% lime mud) are classed as mudstones by Dunham and
as micrites by Folk. Rocks with only grains and no mud are classed as sparites by
Folk and grainstones by Dunham. The term sparite implies that sparry cement occu-
pies intergranular pores. Unconsolidated sediments were excluded by Folk because
his scheme was devised for lithified limestones. Between high and low mud content

are the carbonates with variable proportions of mud and grains. Folk chose the
terms “ sparse ” and “ packed ” to modify micrites with 10% – 50% and over 50%
grains, respectively. By doing so, he set the requirement that grain percentage deter-
mines the rock name. Dunham chose a different approach, probably because he
saw that less than 50% of irregularly shaped grains could create a self - supporting
fabric; hence the origin of the term packstone . He resorted to grain percentage
as the determining factor for naming muddy rocks with grains but without a self -
supporting grain fabric. These muddy, grainy mixtures in which the grains are widely
dispersed ( “ floating ” ) in the mud matrix are termed wackestones . Most industry

professionals use the Dunham classification today because the terms are shorter,

and easier to log when working on large quantities of rock, they do not require
tedious counting or percentage estimates, and they seem to evoke mental images of
rock properties that can be related to reservoir properties.

For the reservoir analyst, detrital rock classifications based on depositional
texture are the most practical and easiest to use. First, the muddy rocks can be
assumed to have formed in environments where winnowing was insignifi cant and
rocks with high grain content represent environments with extensive winnowing,
the “ high - energy ” environments. Second, excluding diagenesis, fracturing, and
special forms of intragranular porosity, mud content is inversely related to inter-
granular porosity. Grainstones and packstones have the highest percentage of