1.
n. [Formation Evaluation]
The ratio of the
resistivity of a
rock filled with water (R
_{o}) to the resistivity of that water (R
_{w}). G.E. Archie postulated that the
formation factor (F) was a constant independent of R
_{w} and solely a function of
pore geometry (the Archie equation I). It has since been shown that F is independent of R
_{w} only for a certain class of petrophysically simple rocks (Archie rocks). In rocks with conductive minerals, such as
shaly sands, there is a more complex dependence. In such cases, the ratio R
_{o}/R
_{w} is known as the apparent formation factor and may vary with R
_{w} , temperature and the type of ion in solution. The intrinsic formation factor is then defined as F corrected for the effect of
shale, or else the value of R
_{o}/R
_{w} at the limit of high salinity (low R
_{w} ). The correction for the effect of shale depends on the
saturation equation used, for example Waxman-Smits, dual water, SGS or CRMM. Unless otherwise stated, the term formation factor usually refers to the apparent formation factor.
F has been related to
porosity (
phi) by several formulae (Archie, Humble and others) that have the general expression F =
a /
phi^{m}, where
a is a constant and
m the porosity exponent.