Page 277 - Petrophysics 2E

250 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

shaly-sand interpretation can be very difficult. Sand interpretation
problems, however, are not all caused by the presence of clays, and
low-resistivity pay zones are not necessarily shaly sand problems.

GENERALIZED SHALE RELATIONSHIP

Based on laboratory investigations and field experience, it has been
found that, generally, alI three forms of clay distribution exist in the same
formation, and that the best formula for finding Sw in shaly sandstones is
of the following general form:

AS: +BS, + c = o (4.89)

where A denotes the combined effect of the amount of sand, its porosity,
cementation, and the resistivity of the saturating water. A always reduces
to the Archie saturation equation, i.e., Equation 4.54, when the shale
volume, Vsh, is zero. B denotes the combined effect of the amount
of shale and its resistivity; C is the reciprocal of the total resistivity of
the shaly sand system. For the range of S, values encountered in the
reservoirs, Equation 4.89 can be expressed as follows:

(4.90)

Based on what is implicitly being practiced in the field, a sandstone
may be considered shaly only if the effective shale (clay) content, Vsh, is
greater than 10% [24]. For Vsh = 0, Equation 4.90 becomes:

1
(S) - = 0 (4.91)
S:
which is equivalent to Equation 4.54, where the saturation exponent
n = 2. If 0 < Vsh 5 10, the contribution of the content of clays to the
term A of Equation 4.89 is sometimes negligible, and the following form
of Equation 4.90 is recommended:

1
Rt
(p)s$+(z)sw --- -0 (4.92)

The positive root of this quadratic Equation 4.90 gives the water
saturation of most shaly sandstones independent of the distribution of
the shale:

(4.93)