As 5 of 11 major oil and gas fields found during the last 15 years in Japan have volcanic-rock reservoirs, they have become an important objective in exploration for petroleum in Japan.

Japanese oil and gas fields have been found mainly in Neogene sedimentary basins developed on the Japan Sea coast along the northern half of Honshu. The basin, associated with volcanic activities, began its depression in the early Miocene, but it continued to subside through the Neogene and Quaternary.

Volcanic-rock reservoirs are present in the formations deposited during middle Miocene and early Pliocene times. They are composed of liparitic, dacitic and/or andesitic lava, agglomerate, and tuff breccia. Intergranular pores are the main cause for the porosity, but many fractures and vugs which may provide additional porosity are known.

Volcanic-rock reservoirs have a rough resemblance to carbonate-rock reservoirs in that fractures and vugs are predominant, formation resistivity is higher than surrounding formations, and the shapes of volcanic-rock masses commonly show reeflike forms. However, the decisive difference between them is that whereas carbonate rocks may be source rocks as well as reservoir rocks, the volcanic rocks are not source rocks. Therefore it is important that, in searching for petroleum in volcanic-rock reservoirs, source rocks must be confirmed close by.

Each volcanic-rock reservoir has been found to have its own pore continuity. Some reservoirs have good pore continuity but others do not. For development of these fields, the difficulty is in determining the location and magnitude of the lava bodies which have good porosity.

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