Page 35 - Geology of Carbonate Reservoirs

16 CARBONATE RESERVOIR ROCK PROPERTIES

WENTWORTH GRAIN SIZE
CLASSIFICATION
Size in mm Particle Name Aggregate Name

Boulder

256 mm
Cobble
Gravel
64 mm
Pebble

4 mm
Granule
2 mm

Sand Sand

1/16 mm
Silt
1/256 mm Mud
Clay

Figure 2.1 The Wentworth grain size classification. Note that all particles finer than sand

1

( 16 − 2.0 mm) are included as mud and all particles coarser than sand are included as gravel.
Much of this “ micrite ” is actually calcisiltite , or silt - sized (62 μ m to 3.90 μ m) sedi-
ment. Note that chalk is a special rock type that is not generally classified as micrite

or 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 down
to the sea floor as disk - shaped particles 2 – 20 μ m in diameter (Milliman, 1974 ). Elec-

tron micrographs of chalk show grain - supported depositional textures without a
matrix of aragonite or calcite crystals finer than the cocoliths; therefore chalk is not

strictly 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 regimes
in carbonate depositional environments as it is with terrigenous sandstones nor is
grain size consistently related to carbonate reservoir porosity or permeability. Car-
bonates consist mainly of biogenic particles that owe their size and shape to skeletal
growth rather than to a history of mechanical transport, deposition, and arrange-
ment. Most carbonate grains originate in the marine environment where waves and
currents fragment, winnow, and sort sediment, primarily along strand plains and
on slope changes (usually associated with bathymetric highs) that occur above