In the oil-bearing Tertiary in Akita and Niigata oil fields, zeolites bearing formations are widely distributed. Zeolites such as clinoptilolite and analcime are alteration products from volcanic glasses, and the latter is formed from the former in the relatively higher diagenetic stage. Their existing stratigraphic horizon seems generally coincident with the alteration stage of montmorillonite to illite and oil producing horizon in rough idea. The famous studies of Burst (1969) and Perry & Hower (1972) in respect to the dehydration of montmorillonite and their contribution to the primary migration of oil in the deeply buried formation where no absorbed water could exist are well known. In this study abovementioned dehydration mechanism is likely recognized in the alteration of clinoptilolite to analcime, in which nearly 1.6 times volume of water produced by montmorillonite dehydration could be expected in the deeply buried horizon. Owing to the remarkable existence of clinoptilolite and analcime, and the obscure alteration stage to illite from montmorillonite in the oil-bearing Tertiary in northeastern Honshu, Japan, the alteration stage of clinoptilolite to analcime seems important in search for oil from the view point of carrier water for the migration of oil in deeply buried formation, porosity anomaly, micro-fracturing, abnormaly high formation pressure and recreation of favorable porosity for the reservoir characteristics.