SP logs are widely used for logging wells because they provide information on lithology and salinity of interstitial water. What two properties do SP logs primarily indicate?

SP logs are widely used for logging wells because they provide information on lithology and salinity of interstitial water. What two properties do SP logs primarily indicate?

• A. Porosity and permeability.
• B. Lithology and porosity.
• C. Lithology and salinity of interstitial water.
• D. Density and thermal conductivity.

Answer: (C)

Spontaneous potential logs rely on natural electrical potentials that arise at the borehole wall from differences between the formation water and the drilling mud, with clay minerals enhancing the signal. Because these potentials are driven by both the mineralogy of the rock (lithology) and the salinity contrast between formation water and mud, SP logs are most informative about lithology changes and the salinity of the interstitial water in the formation. Lithology shows up clearly because clay-rich rocks generate larger SP responses due to their charged surfaces, helping distinguish shales from sands. Salinity of interstitial water shows up through the magnitude and direction of the diffusion potentials: a greater contrast in salinity between mud and formation water yields a stronger SP deflection, so the SP curve tracks salinity variations in the pore fluid. Porosity and permeability aren’t directly measured by SP, since SP reflects electrochemical conditions related to mineralogy and pore-water salinity rather than pore space volume or flow properties. Density and thermal conductivity are obtained from separate logs designed to measure those properties. In short, SP logs primarily indicate lithology and the salinity of interstitial water.

Spontaneous potential logs rely on natural electrical potentials that arise at the borehole wall from differences between the formation water and the drilling mud, with clay minerals enhancing the signal. Because these potentials are driven by both the mineralogy of the rock (lithology) and the salinity contrast between formation water and mud, SP logs are most informative about lithology changes and the salinity of the interstitial water in the formation.

Lithology shows up clearly because clay-rich rocks generate larger SP responses due to their charged surfaces, helping distinguish shales from sands. Salinity of interstitial water shows up through the magnitude and direction of the diffusion potentials: a greater contrast in salinity between mud and formation water yields a stronger SP deflection, so the SP curve tracks salinity variations in the pore fluid.

Porosity and permeability aren’t directly measured by SP, since SP reflects electrochemical conditions related to mineralogy and pore-water salinity rather than pore space volume or flow properties. Density and thermal conductivity are obtained from separate logs designed to measure those properties.

In short, SP logs primarily indicate lithology and the salinity of interstitial water.