Will a SP log spike lower or higher in a reservoir containing hydrocarbons?

Will a SP log spike lower or higher in a reservoir containing hydrocarbons?

• A. Lower
• B. Higher
• C. Zero
• D. Cannot be determined

Answer: (B)

Spontaneous Potential (SP) logs capture natural electrical potentials that arise from ion diffusion at the mud–formation interface, especially where clays are present and formation water salinity differs from the drilling mud. When hydrocarbons occupy the pore space, they replace conductive formation water with a non‑conductive fluid, which reduces the pathway for ions to leak between the mud and the formation. That reduction in leakage current makes the diffusive electrochemical potential more pronounced, so the SP deflection becomes larger. In other words, the SP signal tends to spike higher in a hydrocarbon-bearing interval. The exact sign can depend on electrode polarity, but the general trend is a higher SP anomaly where hydrocarbons are present.

Spontaneous Potential (SP) logs capture natural electrical potentials that arise from ion diffusion at the mud–formation interface, especially where clays are present and formation water salinity differs from the drilling mud. When hydrocarbons occupy the pore space, they replace conductive formation water with a non‑conductive fluid, which reduces the pathway for ions to leak between the mud and the formation. That reduction in leakage current makes the diffusive electrochemical potential more pronounced, so the SP deflection becomes larger. In other words, the SP signal tends to spike higher in a hydrocarbon-bearing interval. The exact sign can depend on electrode polarity, but the general trend is a higher SP anomaly where hydrocarbons are present.