SP logs are used to infer which properties of subsurface formations?

SP logs are used to infer which properties of subsurface formations?

• A. Porosity and permeability.
• B. Lithology and salinity of interstitial water.
• C. Density and magnetic susceptibility.
• D. Temperature and pore pressure.

Answer: (B)

Spontaneous potential logs detect natural electrical differences that arise at the borehole wall from diffusion and electrochemical interactions between the drilling mud and the formation water. These potentials are strongly influenced by two real-world factors: the lithology of the rock (especially clay-rich layers that conduct electricity and influence surface charge) and the salinity contrast between the formation water and the drilling mud. Because of that, SP logs are best used to infer what the rock is made of (lithology) and whether the water occupying the pores is salty or fresh (the salinity of interstitial water). In practice, clay-rich zones produce distinctive SP responses due to their surface conduction, and changes in formation water salinity relative to the mud shift the baseline and the magnitude of SP signals at bed boundaries. SP readings aren’t primarily used to quantify porosity or permeability, and they don’t directly measure density, magnetic properties, temperature, or pore pressure, which involve different physical mechanisms and logging tools.

Spontaneous potential logs detect natural electrical differences that arise at the borehole wall from diffusion and electrochemical interactions between the drilling mud and the formation water. These potentials are strongly influenced by two real-world factors: the lithology of the rock (especially clay-rich layers that conduct electricity and influence surface charge) and the salinity contrast between the formation water and the drilling mud. Because of that, SP logs are best used to infer what the rock is made of (lithology) and whether the water occupying the pores is salty or fresh (the salinity of interstitial water).

In practice, clay-rich zones produce distinctive SP responses due to their surface conduction, and changes in formation water salinity relative to the mud shift the baseline and the magnitude of SP signals at bed boundaries. SP readings aren’t primarily used to quantify porosity or permeability, and they don’t directly measure density, magnetic properties, temperature, or pore pressure, which involve different physical mechanisms and logging tools.