Page 36 - Geology of Carbonate Reservoirs

FUNDAMENTAL ROCK PROPERTIES 17

fair - weather wave base. However, carbonate grains are produced in a wide variety
of environments; consequently, it is risky to use grain size and shape alone as indica-
tors of the hydrologic regime. It is better to consider grain size, sorting, grain shape,
amount and type of grain fragmentation, and mechanical durability of the grains as
clues for interpreting depositional environments. Some grains are produced by bio-
erosion — boring, rasping, grinding, and digesting carbonate constituents by organ-
isms. The biological reworking makes grain size and shape virtually useless as
indicators of the hydrologic regime. Perhaps the most effective villain in altering
carbonate grain size and shape is diagenesis. Micrite can be produced from sand
and gravel, grains can be enlarged by cement overgrowths, and shape can be changed
by cementation or dissolution. Fortunately, diagenetic changes are not diffi cult to
identify in thin sections. Sorting and grain size (Figure 2.2 ) are textural attributes
that can be useful in studying carbonate rocks because they infl uence depositional
porosity and permeability. Porosity is independent of grain size where grains are
ideal spheres, but permeability varies with particle size because small grains have
small intergranular pores with small pore throats. Sorting and grain packing are also

strongly related to permeability because sorting and packing influence the geometri-
cal relationship between pores and pore throats. A high correlation exists between
permeability and pore throat dimensions, as we will see later in our discussions on
capillary pressure, permeability, and reservoir quality, but pore geometry alone is
not strongly related to permeability. It is the pore – pore throat relationship that is
so important.
Mechanical abrasion, along with hydraulic size and shape sorting, are important
processes in beach, dune, and shallow, slope - break environments where mud - free

Median Grain Size – mm
1.0 0.5 0.25 0.125 0.064 0.044
1.0 POROSITY – % Extremely
PERMEABILITY – DARCYS 200 100 50 25 40 10 Very Well
1.1 400 42 Well

Sorting – S o 1.4 32 34 36 38 5 2.5 Well Sorting
1.2

Moderate
2.0
28 30 1.0 0.5 Poor
2.7
26
Very Poor
5.7
Coarse Medium Fine Very Fine Silt
Grain Size

Figure 2.2 A plot showing the relationships between grain size, sorting, and porosity in

unconsolidated sands. Based on data in Beard and Weyl (1973) and adapted from unpub-
lished notes with permission from R. M. Sneider (1988) . Note that porosity does not vary
with grain size but does vary with sorting. Permeability varies with both grain size and
sorting.