Page 272 - Petrophysics 2E

RESISTIVITY OF SWY (CLAYEY) RESERVOIR ROCKS 245

factor m = 2, the saturation exponent n = 2, and the constant
a= 1:

(4.73)

The shale volume Vsh is determined from various well logs which are
considered to be good clay indicators [22].
(2) Dispersed clays, which evolved from the in situ alteration and
precipitation of various clay minerals, may adhere and coat sand
grains or they may partially fill the pore spaces. Figure 4.30
shows three types of dispersed shale in a sandstone bed. This
mode of clay distribution considerably reduces permeability and
porosity of the formation, while increasing water saturation. This
increase in S, is due to the fact that clays adsorb more water than

Figure 4.30. Types of dispersed shale: (a) dkcrete-particle kaolinite, (b) pore-lining
chlorite, and (c) pore-bridging illite [20].