Which term represents the ratio Ro/Rw explicitly?

• A. Resistivity index
• B. Formation factor
• C. Capillary factor
• D. Permeability factor

Answer: (B)

Formation factor is the ratio of how resistive the rock formation is to the resistivity of the pore fluid. When a rock is saturated with formation water, its bulk resistivity is Ro, and the resistivity of that same water (the brine) is Rw. The quotient Ro/Rw gives the formation factor, a measure of how the rock’s pore structure affects electrical flow. This factor encapsulates porosity and tortuosity and is central to Archie's law, where Ro/Rw ≈ a φ^-m for clean rocks, linking rock texture to electrical response. It’s the term that explicitly represents this ratio, unlike resistivity index (a different ratio tied to saturation changes) or the other named factors.