Page 317 - Petrophysics 2E

288 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

If the sand contains petroleum, the true resistivity factor Rt will
increase, whereas the formation factor FR will remain the same because
it is a function of the formation porosity only. Therefore, Equation 4.145
is of the general form

(4.146)

where Rwa is the apparent water resistivity. Thus, if Rwa is calculated
from Equation 4.146, one would actually be calculating a value of Rwa =
Rw/Sk . Then, if Rw is either experimentally measured from a sample or
calculated from a chemical analysis using Figure 4.4, one can solve for S,.
The value of Rm, therefore, can be used to select hydrocarbon-bearing
zones.
The following analytical procedure is recommended for selecting sand
beds containing hydrocarbons [48].
Consider two adjacent porous and permeable zones: zone 1 containing
hydrocarbons and zone 2 is 100% water saturated. Comparing the Rwa
values of the two zones gives:

(4.147)

Inasmuch as Rw is constant regardless of the value of S, the water
saturation in zone 2 is loo%, i.e., 1R2 = 1, and Rwa2 is the true R, or
a minimum observed value @,a)-, one obtains:

(4.148)

Solving for S, for any zone:

(4.149)

Assuming Rw remains fairly constant along the zones of interest, values
of R,, can be determined for each zone and compared. Zones where
Rwa is greater than approximately 4Rw generally have S, less than 50%
and should be considered potential hydrocarbon-bearing zones. Hilchie
proposed an elaborate but complete and practical algorithm for selecting
hydrocarbon-bearing zones using the R,, method [3]. According to this
algorithm, all zones where R,, > 3R, have S, < 60% and are potentially
hydrocarbon bearing.