Nucleons which are chemically bound to oil and water in geological formations possess magnetic moments which align in the earth's magnetic field causing the natural nuclear paramagnetism of the formation. An A.C. magnetic field at the Larmor frequency is repetititively applied to the formation adjacent to a borehole resulting in a repetitive excitation-relaxation process of the nucleons present within an "excitation zone" which results in an amplitude modulation of the paramagnetism within this zone. This physical process results in an amplitude modulation of the earth's magnetic field in the vicinity of the excitation zone of the formation and is called the Paramagnetic Logging Effect (PLE). The PLE may be measured with logging tools in open holes and within cased well bores. The PLE measures the following quantities within a predetermined excitation zone of the formation: amount of oil, amount of water, total fluid volume, the viscosity of oil present, oil saturation and water saturation factors, estimations of the permeability, positions of vertical oil and water boundaries adjacent to the borehole, and the locations of lateral discontinuities of the oil bearing formation. The PLE is sensitive to such parameters at least 10 feet laterally into formation and up to perhaps 100 feet radially away from the drill hole. A thorough theoretical description of the PLE in open holes and in the cased hole environment is presented along with the detailed engineering data necessary for the construction of practical tools. Another variety of the PLE is based on the repetitive excitation-relaxation process of natural electronic paramagnetic impurities found within petroleum. Measurement of the presence of such impurities indicate the presence of petroleum in formation.