Volcanic Reservoirs, Their Characteristics of the Development and Production
Authors: Ukai Mitsuo , Katahira Tadami , Kume Yoichi , Yamazaki Susumu 
Published: December 2007

Introduction
Japanese oil and gas fields have been found mainly in the northern half of the country. Especially, the Japan Sea coastal area accounts for about 99% of Japanese petroleum production.
Japanese oil and gas fields, except for those gas fields of water dissolved type, occur in the Neogene strata.
In this paper, the writers will describe an outline of Japanese oil and gas fields and then explain in detail about those which have volcanic or phyroclastic reservoirs. phyroclastic reservoirs. Generally speaking, igneous rocks are inferior to sandstones and carbonate rocks as reservoir rock. Knebel and Rodriguez-Eraso (1956) made a statistical study of 236 major oil fields of the free world and stated that "…..sand reservoirs hold 59 per cent of oil found in major fields, with carbonates holding almost the entire remaining 41 per cent. Other fractured rocks such as shales and igneous or metamorphic rocks contain only 0.8 per cent of the oil in major fields."
However, it is true that igneous rocks can form payable reservoir in some places. For example, Thrall oil field in Texas, Lytton Springs oil field places. For example, Thrall oil field in Texas, Lytton Springs oil field in Texas, Dinehbi-Keyah oil field in Arizona and Djatibarang Shutin gas field discovered recently in Indonesia are famous for the reason that the reservoir is igneous.
As 5 of 11 major oil and gas fields found during the last 15 years in northeast Japan have igneous reservoirs, reservoirs of this type has recently become important in the area.

General Geology and Outlines of Oil and Gas Fields in the Japan Sea Coastal Area of Northeast Japan
Oil and gas fields have been found in the Neogene sedimentary basin extending along the coast of the Sea of Japan. The basin began its depression associated with volcanic activities in early Miocene and continued to subside through the Meocene and Quarternary periods. It is about 700 km long and 80 km wide and the thickness of sediments is estimated to be about 10,000 m at the point of maximum depression. This sedimentary basin is generally called Uetsu geosyncline.
Basement rock of Uetsu geosyncline is mostly composed of slightly metamorphosed Paleozoic formations and granitic rocks. Granitic rocks are presumed to have intruded during the late Mesozoic period. Only small presumed to have intruded during the late Mesozoic period. Only small masses of Mesozoic formations are found in the southern part of this basin but no Paleogene formations are found in this area.
Uetsu geosyncline is, as shown in Fig. 1, subdivided into two subbasins. The northern one is Akita subbasin and the southern one is Niigata subbasin.
They are similar in their goelogical development, however, some differences in the detail of sedimentary facies, amount of sediments, mode and period of volcanic activities and tectonic movements can be seen.
Noegene successions of these subbasin are shown in Table 1. Formations in these subbasins can be correlated in detail chiefly using plaktonic and benthonic foraminifers.
As shown in Table 1, oil and gas fields in these subbasins occur in the formations deposited during middle Miocene and early Pliocene.
Most of the oil and gas fields in this area have developed on anticlinal structures with stratigraphic traps such as thinning out of sandstone. However, pure stratigraphic trap has not yet been found here.