formation factor — Oil & Gas Glossary
The ratio of the resistivity of a rock filled with water (Ro) to the resistivity of that water (Rw). G.E. Archie postulated that the formation factor (F) was a constant independent of Rw and solely a function of pore geometry (the Archie equation I). It has since been shown that F is independent of Rw only for a certain class
Detailed Explanation
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 Ro/Rw is known as the apparent formation factor and may vary with Rw , 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 Ro/Rw at the limit of high salinity (low Rw ). 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 / phim, where a is a constant and m the porosity exponent.