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Petrography and Texture of Petroleum-Bearing Formations 59

0.1 1 10 1000
Pore Thraal Radius, microns
Figure 3-8. Typical bimodal pore throat size distributions in porous formation
(after AI-Mahtot and Mason, ©1996; reprinted by permission of the Turkish
Journal of Oil and Gas).

where D is the diameter of the pores approximated by spheres, D m, is
the mean pore diameter calculated by:

D= DF(D)dD (3-17)

and s d is the standard deviation, and D min and D max denote the smallest
and largest diameters.

Bi-Modal Distribution. Typically, the pore body and pore throat sizes
vary over finite ranges and the size distributions can display a number
of peaks corresponding to various fractions of pore bodies and pore
throats in porous media. If only two groups, such as the fine and coarse
fractions, are considered, a bi-modal distribution function according to
Popplewell et al. (1989) can be used for mathematical representation of
the size distribution:

(3-18)