Zeolitic Alteration of Vitric Tuffs in the Akita Oil Field (Part 3) On the Noshiro sedimentary basin

Abstract

Zeolitic alteration of vitric tuffs from the Neogene oil-bearing deposits of the Noshiro basin (Fig. 1) in the Akita oil field shows regular zonal arrangements, in descending order as (Fig. 2), fresh glass zone-clinoptilolite zone-analcime zone-authigenic albite zone. Each zeolite zone considerably varies with the localities (Figs. 3 and 4) in both stratigraphic horizons and thickness.The natural densities of the mudstone at the top of the clinoptilolite zone is measured to be 1.87± and that of the analcime zone 2.15± (Fig. 5). From the thickness of the strata (Fig. 6) above the mudstone, the initial thickness before burial is calculated to be 830m+ above the clinoptilolite zone and 1500m+ above the analcime zone.The zeolitic alterations of the vitritic tuffs in the Noshiro basin are interrelated to their overburden thickness (Figs. 7-11). Such differences of sedimentation are inferred to be due to the differential movements of the sedimentary basin caused by the Dewa Disturbance during the later Funakawan and the early Tentokujian stages.