an apparatus and method of measuring a parameter characteristic of a rock formation in an oil well is provided with a device for generating a sensing field within a volume of the rock formation and a device for causing a flow through the volume in the presence of the sensing field , further including sensors responsive to changes in the volume , wherein a sensor response is indicative of the amounts of fluid , particularly hydrocarbon and water saturations and irreducible hydrocarbon and water saturations . measurements can be made before the flow affects the measuring volume and after onset of the flow through the measuring volume .