Page 51 - Geology of Carbonate Reservoirs

32 CARBONATE RESERVOIR ROCK PROPERTIES

According to Berg ’ s calculations, pore diameter in a sample with about 30% poros-
ity and well - sorted spherical grains varies from about 0.4 D to 0.06 D ( D = grain
diameter), depending on packing: cubic packing has the largest pore diameter, while
rhombohedral has the smallest. High porosity grainstones with cubic packing and
near - spherical grains will have pore sizes in the 0.06 – 0.4 - mm range for 1 - mm diam-
eter ooids. Porosity does not vary with grain diameter, but it does vary with packing
and sorting. Cubic and rhombohedral packing arrangements for idealized, spherical
grains have 47.6% and 25.96% porosity, respectively (Figure 2.9 ). If a second size
of spherical grains is introduced, that is, if sorting is poor (Figure 2.10 ), the porosity
of a cubic - packed sample is reduced from about 48% to about 13%. Grain shape
also affects porosity. A comparatively small number of irregularly shaped grains,

Cubic packing Rhombohedral packing

Figure 2.9 The influence of grain packing on porosity. Cubic packing of spherical grains has
47.6% porosity regardless of grain size, but rhombohedral packing of the same grains reduces
porosity to 26.9%. Pores, shown below the grain packs, are “ negative images ” of the grains.
In the cubic packing example it is clear that each pore is connected to others by three pore
throats. Later this will be known as a pore system with a coordination number of 3.

Cubic packing Rhombohedral packing
f = 47.6% f = 26%

Cubic packing
Two grain sizes f ≈ 12.5%

Figure 2.10 Cubic and rhombohedral packing and the importance of sorting. When a second,

smaller but uniform grain size is introduced, the original 47.6% porosity is reduced to
12.5%.