Reservoirs are shown as deflections (either positive or negative) from a relatively stable (and arbitrary) shale baseline. The direction of the deflection is determined by the relative salinities (resistivities) of the formation water (Rw) and the mud filtrate (Rmf), and is not directly related to formation porosity or permeability.
As a rule of thumb the following relationships are true:
If Rmf > Rw, then the SP deflection is negative.
If Rmf = Rw, then the SP deflection is zero.
If Rmf < Rw, then the SP deflection is positive.
As a rule of thumb the following relationships are true:
If Rmf > Rw, then the SP deflection is negative.
If Rmf = Rw, then the SP deflection is zero.
If Rmf < Rw, then the SP deflection is positive.
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