日本ゼオライト学会　刊行物 Publication of Japan Zeolite Association

Zeolite minerals commonly occur in Tertiary formations in Japan. Some were formed in the proceeding of burial with the increase of ground temperature in the presence of formation water. Others were formed by hydrothermal water originated from magmatic activity. There occur zeolite minerals also in oil and gas bearing sedimentary basin. The occurrence of each zeolite mineral in these basins has been used to estimate the thermal gradient of present or past time, However, there are some occurrences recently studied, in which zeolite minerals have different significances in oil and gas bearing sedimentary basin. In the MITI Sanrikuoki borehole drilled offshore Iwate Prefecture, the occurrence and origin of laumontite was investigated and interpreted as follows. The laumontite in this borehole occurs in the dissolution void after calcite cement. The formation water was mixed with organic acid in the depth, migrated upward, altered by the reaction with minerals and calcite cement and finally saturated with laumontite. Secondary porosity formed after dissolution of calcite cement is only partly filled with laumontite, and a good reservoir property persists. In Yufutsu oil and gas field, Hokkaido, laumontite veins abundantly occur in the fractured granitoid and conglomerate reservoirs. By detailed observations and analyses, the laumontite-cemented fractures were reopened under the duplex movement and provided fair pore space for later accumulation of oil and gas.