This log measures the bulk resistivity (the reciprocal of conductivity) of the formation. Resistivity is defined as the degree to which a substance resists the flow of electric current. Resistivity is a function of porosity and pore fluid in a rock. Porous rock containing conductive fluid (such as saline water) will have low resistivity. A non-porous rock or hydrocarbon-bearing formation has high resistivity. This log is very useful for determining the type of fluids in formations and is frequently used as an indicator of formation lithology and grain size.