Deposition of marine sediments has occurred in different parts of Japan and its offshore areas at intervals from early Paleozoic time to the present. Most rocks of the Paleozoic sedimentary basins are now complex in structure and many have been metamorphosed; no indications of oil have been discovered in these rocks and they hold little promise for the future. Mesozoic sedimentary rocks are also complex in structure. Although no oil has been produced from them, a limestone of Jurassic age is slightly petroliferous and oil seeps are known from Cretaceous rocks in Hokkaido.

During Tertiary time thousands of meters of marine sediments were deposited in a geosyncline which extended along the western part of northern Japan. In late Pliocene and Pleistocene time crustal movements separated the Tertiary rocks into several basins. Petroleum exploration is presently concentrated in these Tertiary basins and their extensions offshore beneath the Japan Sea. Most oil so far discovered is trapped in small anticlinal structures which trend north-northeast.

Oil production in Japan was first officially, recorded in 1874. Production in 1891 was 63,523 barrels. Development continued and in 1936 production reached 2,457,503 barrels. Much of this oil came from the Yabase field which was discovered in 1934. After 1939 production fell.

Exploration received new impetus during the American Occupation of Japan, especially through the leadership of Hubert G. Schenck. Deeper producing horizons were discovered in old fields, including the Yabase, and study and exploration of untested areas, particularly in Hokkaido, were undertaken.

Current exploration is carried on principally by the Japan Petroleum Exploration Co., Ltd., and the Teikoku Oil Co. The former is drilling anticlinal structures that were located by seismic surveys off the northwestern coast of Honshu. Tests of an offshore well completed in 1960 produced 1,800 b/d. Teikoku Oil Co. has concentrated its exploration program on the extension of proved fields. Total production has increased and 3,675,047 barrels of petroleum were produced in 1960.

Future exploration will include additional offshore drilling, testing of deeper formations in proved fields, and further exploration of unproved areas of Cretaceous and Tertiary sediments.

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