Page 269 - Petrophysics 2E

242 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

Combining this expression with Equation 4.60 for a nonuniform
grain-size distribution system, the following equation is obtained:

(4.64)

This and similar empirical relationships are useful because they dow
one (a) to understand, qualitatively and quantitatively, the interaction of
various petrophysical parameters, and (b) to approximate the formation
permeability in uncored wells knowing porosity and formation resistivity
factor, which can both be derived from well logs. The value of 02 varies
between zero, for a system with uniformly distributed grain-sizes, to unity
for the theoreticaI case in which every single grain has a different size. For
a laboratory sandpack, 02 is approximately equal to zero and Equation
4.64 becomes:
k = 1.536 x loz4 ( 1 (4.65)

For 62 = 1, the term e1.6502 in Equations 4.60 and 4.64 is equal to
5.2. Thus, the maximum effect of grain-size distribution is to reduce
the permeability of the ideally uniform system (in which 02 = 0) by
approximately five-fold. Typical values of the standard deviation term
os varied from 0.35 to 0.65 in the Dakota Sandstone outcrop near
Cortez, Colorado, and from 0.40 to 0.75 in the Gallup Formation outcrop
near Farmhgton, New Mexico. These outcrops are considered to be
relatively clean consolidated sands and are not significantly modified by
groundwater circulation.

RESISTIVITY OF SHALY (CLAYEY) RESERVOIR ROCKS

The presence of conductive clays and shales considerably complicates
the interpretation of resistivity data of partially saturated formations. The
shale type, the percentage present, and the mode of distribution in the
formation have different effects on the resistivity. Generally, however,
the presence of clay or shale in a sand bed lowers the true formation
resistivity Rt and, if not corrected, will result in overestimating S,
i.e., interpreting as water-bearing zones that are actually oil-bearing.
Shales contain, in various proportions, clay minerals such as illite,
montmorillonite, and kaolinite, as well as silt, carbonates, and other
nonclay minerals. Silt is a very fine-grained material that is predominantly
quartz, but may include feldspar, calcite, and other minerals. The silt