Well log evaluation for fluid hydrocarbons has always drawn upon petrophysical relationships for the aquifer zone where the water saturation is unity and the resultingly less complex petrophysical regime can be used for control. This dependence upon aquifer petrophysics has traditionally centred on the Archie equations which are strictly valid only for clean sands. Research directed at the establishment of more generally applicable equations for the evaluation of clean or shaly hydrocarbon reservoirs has again been based initially on the aquifer zone to avoid water saturation as a complicating parameter. These equations, which conventionally draw upon electrical resistivity concepts, are demonstrably equivalent to those used in the water industry. It is proposed that they form part of a larger family of petrophysical relationships which describe the electrical behaviour of water-saturated reservoir rocks as a whole. This behaviour is seen to be consistent for arenaceous core plugs representing a wide range of petrophysical characteristics and it might therefore describe a type pattern which is retained through to the hydrocarbon zone.