Formation factor, F. Equal to the ratio of the [[resistivity]] of the l00% water-saturated rock framework to the resistivity of the water solution contained in the rock. The limiting formation factor is an intrinsic characteristic of the rock, obtainable with reliability only when the interpore water solution is highly salt saturated. The apparent formation factor. most often obtained, is a function of [[porosity]], [[salinity]] of water filling the pores, pore geometry, clay content, and presence of electrically conductive solid matter. See [[Archie's formulas]].