The zeolitic alteration of vitric tuffs in the Neogene oil-bearing formations in Akita oil field was studied. Zeolitic zonation at Yuri hilly land is shown in Fig. 1, and Table 2. Zeolitic columnar sections of underground rocks at lowland are shown in Figures 2 and 3. Zeolitization of underground vitric tuffs at the lowland area more moved upwards in geologic horizen as compared with that of the hilly land. Two different types of alteration facies, the upheaval
type and the depression type, are distinguished among the underground tuffs, as showing in the following table:
Marginal area of the Akita sedimentary basin, in which Yuri hilly land is situated, were widely and largely upheaved by the Dewa disturbance. After this disturbance, the physical conditions under which vitric tuffs had been altered into zeolite was diminished in these upheaved area. In the remained
basin, where the depression was succeeded, the zeolitization upwardly somewhat increased. During the Tentokujian stage, the sedimentary basin was differentiated into raised belts and depressed zones. At the raised belt the favorable condition for zeolitization gradually decreased. While, in the depressed zone younger formations successively deposited and increased the zeolitic alteration of vitric tuffs until the end of the Sasaokan stage.
In Akita oil field, pyroclastic rocks, of which acidic tuffs are most extensive, are commonly intercalated in oil-bearing formations, and they often formed oil and natural gas reservoirs.
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