Page 44 - Geology of Carbonate Reservoirs

CLASSIFICATION OF CARBONATE ROCKS 25

Over 2/3 lime mud matrix Subequal Over 2/3 spar cement
Percent spar and
allochems 0 - 1% 1 - 10% 10 - 50% Over 50% lime mud Sorting Sorting Rounded
and
good
poor
abraded
Representative Micrite and Fossili- Sparse Packed Poorly Unsorted Sorted Rounded
rock terms dismicrite ferous biomicrite biomicrite washed biosparite biosparite
micrite biosparite biosparite

Micrite Sparry calcite cement

Figure 2.6 The classification scheme for detrital carbonate rocks developed by R. L. Folk

first published in 1959 and later in 1962. Folk classified biogenic (reef) carbonates as bio-
lithites. He did not include a term for completely recrystallized or replaced rocks. (Adapted
from the illustration in Folk (1962) .)

Depositional Texture Recognizable Depositional
Texture Not
Original Components Not Bound Together During Deposition Original components
were bound together Recognizable
Contains mud (particles of clay and Grain-supported during deposition, as
fine silt size, less than 20 microns) shown by intergrown (Subdivide according
skeletal matter, to classifications
designed to bear
Mud-supported Grain-supported lamination contrary to on physical texture
gravity, or sediment-
floored cavities that are or diagenesis.)
Less than More than More than * Less than * roofed over by organic
10 percent 10 percent 10 percent 10 percent or questionably organic
grains grains mud mud matter and are too large
to be interslices.
Crystalline
Mudstone Wackestone Packstone Grainstone Boundstone Carbonate

* Modification of original Dunham classification by changing percent mud from 1 to 10%

Figure 2.7 R. J. Dunham ’ s classification of carbonate rocks. Note that the classifi cation
includes detrital carbonates as mudstones through grainstones, biogenic (reef) carbonates as
boundstones, and diagenetically altered carbonates as crystalline carbonate. (Adapted from

the classification scheme illustrated in Dunham (1962) .)

fundamental rock properties systematic and reproducible, and (2) to facilitate infor-
mation transfer. Reservoir geologists are concerned with both aspects, but they are
even more concerned with relating rock classifications to reservoir properties. In

order for that to be possible, reservoir and rock properties must have characteristics
in common. That is, rock classifications correspond closely with reservoir properties

only if reservoir properties depend on parameters used in the rock classifi cations.
For example, a classifi cation of detrital carbonates based on depositional texture is
independent of pore types formed by diagenesis, by fracturing, or by biological
growth patterns in reef rocks. But some indirect relationships may exist between