The main coal deposits of Japan are of Paleogene age. The coals are high-volatile bituminous to subbituminous rank. Their range of carbon-oxygen ratio (O/C) is below and above 0.120, respectively, with a range of carbon-hydrogen ratio (H/C) from 0.650 to 0.900. The Paleogene formations in coalfields consist of marine and nonmarine sedimentary rocks and their thickness varies in different fields. They are developed fully in central Hokkaido and northern Kyushu where their maximum thickness is 4,000 and 2,000 m, respectively.

The deformation of the coal measures is much stronger in central Hokkaido than in northern Kyushu, though coals in both regions are high-volatile bituminous. The deformations in northern Kyushu and Joban-Kushiro are similar, but coal of the latter is subbituminous. These facts strongly suggest that the degree of coalification apparently is not controlled only by depth of burial and tectonic deformation.

Various authigenic zeolites are present in altered silicic vitric tuffs interbedded with coal-bearing formations. Clinoptilolite and mordenite are associated with the host formations of subbituminous coal as in Joban-Kushiro, whereas analcime, heulandite, and laumontite are present with bituminous coal in central Hokkaido and northern Kyushu. These specific assemblages of zeolites and coal ranks are recognized throughout the Japanese coalfields. The zeolites are distributed in a vertically zonal arrangement which is, with depth, clinoptilolite-mordenite, analcime-heulandite, analcime-laumontite, and albite zones. This zonation is well established in the Japanese Neogene oil and gas fields. It is controlled mainly by depth of burial and geothermal gradient, i.e., by temperature. The st bility field of each of the zeolite species was estimated by using deepwell data.

Comparison with zeolite occurrence in the oil and gas fields suggests that the degree of coalification is controlled mainly by temperature, and that the temperature range of coalification of bituminous rank is estimated to be from 85 to 125°C.

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