Which log is used to distinguish permeable zones from non-permeable zones and is measured in millivolts (mV)?

• A. Gamma Ray Log
• B. Neutron Log
• C. SP Logs
• D. Resistivity Log

Answer: (C)

Spontaneous potential (SP) logs measure natural electric voltages in the borehole, recorded in millivolts. This electrical signal arises from electrochemical differences between formation water and drilling mud as fluids interact at the borehole wall. Permeable zones, which allow water flow, create a distinct potential because formation water can exchange ions with the mud filtrate, producing a measurable deflection on the SP curve. Nonpermeable intervals, such as shale, limit this exchange, so their SP response is different or subdued, making SP a useful tool for spotting permeability contrasts between beds. Other logs operate on different physical signals and are not measured in millivolts: gamma ray and neutron logs probe lithology and porosity, while resistivity logs measure electrical resistance in ohm-meters, not millivolts.