Page 276 - Petrophysics 2E

RESISTIVITY OF SHALY (CLAYEY) RESERVOIR ROCKS 249

The concept that shaly or clayey sands can be partitioned into
two volume components gives satisfactory results only for the case
of laminated shaly sands, because the clay content can vary without
affecting the clean sand porosity QCs. However, for the cases of dispersed
and structural shales, two-volume model may not account fully for
variations in clay contents [25]. If a multivolume model is used, it can be
shown that the effective porosity of a shaly multi-component lithology is:

where fmc is the multi-component sensitivity factor, which can be
determined from a zone where Vsh = 1 and @e = 0.

APPROXIMATE SHALE RELATIONSHIP

After comparing several models proposed in the literature, Hilchie [26]
and Fertlan and Hammack [54] developed the following approximate
shaly sand equation which generally is applicable to any shale (clay)
distribution:

where LCs is the cleansand water saturation which can be calculated
from Archie equation:

0.5

where FR reflects the effective porosity, oe, and Fsh is the shale correction
factor [54]

(4.88)

Equation 4.86 illustrates the practical aspect of the shale effect on the
estimation of hydrocarbon in place. If, for instance, vsh is neglected, the
shale correction factor Fsh = 0. This will result in an overestimation of
water saturation, and consequently, an underestimation of hydrocarbon
in place. On the other hand, if Vsh is overestimated, it produces the
opposite effect, i.e. an overestimation of hydrocarbon in place [55].
In general, it is easier to interpret shaly (clayey) formations where
@e and the salinity of water are both very high. Conversely, areas
where @e is low and the water is fresher-such as the Cretaceous
sands in Alberta and the Rocky Mountains, and parts of California-the