Seismic prospecting has been a traditional means in search for oil-gas reser-voirs; nevertheless, it has such disadvantages as complicated -technology, high cost and less success. The a card measurement test performed in a number of oilfields indicates that ²²²Rn can serve as a pathfinder element for oil-gas re servoirs. Its technique is quite simple and its cost is extremely low. Anomali es are characterized by the phenomenon that the sampling points of low inten sity have nothing to do with the lithologic characters and fall to peripheral areas of the oil-bearing district or over the structure. If the effects of struc tural faults are not considered, the boundary of the oilfield is likely to be indi cated by relatively high 222Rn anomalies. This paper makes a tentative discuss ion on formation mechanism and model of the anomaly and physical prerequi site for a card measurement.