When a SP log deflects to the right of the shale base line, what is the most probable formation fluid?

• A. Salt water
• B. Oil
• C. Fresh water
• D. Gas

Answer: (C)

Spontaneous potential (SP) logging compares the natural electrical potential at the borehole wall caused by diffusion between the drilling mud filtrate and the formation water. The shale baseline sits as a reference because shale minerals create a characteristic SP signature. The direction of SP deflection relative to that baseline depends on the salinity contrast between the mud filtrate and the formation water. If the formation water is fresher than the mud filtrate, ions diffuse in a way that shifts the SP curve to the right of the shale baseline. This rightward deflection is the classic indicator of a freshwater formation. In contrast, a saltier (brine) formation water tends to produce a leftward deflection. Oil or gas in the formation doesn’t by itself determine SP direction, so the rightward deflection here points to freshwater as the formation fluid.