Page 266 - Petrophysics 2E

FORMATION RESISTIVITY FACTOR 239

25

20

5

0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
S,, Fraction
Figure 4.25. Resistivity index us. water saturation.

CORRELATION BETWEEN FR AND PERMEABILITY

Three factors are important in correlating FR with the permeability of
sedimentary rocks:

(a) the range of grain-sizes, which is characterized by a grain-size
distribution factor and a geometric mean grain diameter;
(b) the degree of packing of the sand particles, which is a function of
several factors, such as the angularity of the particles, the current
velocity during the deposition, and the grain-size distribution;
and
(c) the combination of cementation and compaction of the sediments.

Of these three factors, only the range of particle sizes can be quantified
with a reasonable accuracy, even though a reservoir core sample is too
small to have a statistical meaning.
Once a sand deposit is accumulated and buried by subsequent
depositions, the texture-controlled permeability of this sand is severely
distorted by compaction and cementing materials, such as calcite, silica,
and various types of clays. Clays generally affect the original distribution
of permeability only slightly, because clays tend to deform by compaction
to conform to the adjacent quartz grains, and seem to be distributed
in a relatively uniform manner throughout the sandy portion of the