ABSTRACT: Petroleum Geology of the Offshore Region, North East Japan. A Model for Producing Forearc Type Basin

TSUDA MUNESHIGE IWAZAKI TETSUHARU, and FUNAYAMA MASAAKI,* Teikoku Oil Company Ltd., Tokyo, Japan

Typical forearc sedimentary basins exist in northeast Japan along the western margin of Pacific Ocean. Joban basin, one of the forearc basin, has its major deposition in the offshore area, and is unique because it is a single basin having commercial hydrocarbon accumulation among forearc basins in Japan. Cretaceous to recent strata distributes in the Joban basin with maximum thickness more than 5000 m. Sediments in the basin are mostly siliciclastic rocks and producing gas reservoirs are middle Miocene sandstones. Another gas-bearing layer has also been found in Oligocene sandstones.

Source rocks are thought to be Paleogene and/or Upper Cretaceous arillaceous rocks having approximately 0.5 wt. % of organic carbon over a wide areal extent. Kerogen compositions of these sediments are predominantly gas-prone, woody-coaly. Another possible origin of gas is coal layers intercalated in the Paleogene-Cretaceous strata.

Major traps are anticlines, associated with faults, formed as a result of uplifting (tectonic movement) during middle Miocene. More than ten structural highs have been recognized between the coastline and a "ridge-shaped" structural high comprising of Cretaceous strata. MITI-Joban Oki, a well promoted by MITI (the Ministry of International Trade and Industry) in 1991, was drilled in the ridge. The principal target was cretaceous rocks, however, gas-bearing Miocene sands were found instead. Hence it is expected that there are some unexplored anticline traps of Miocene sands which overly the ridge unconformably.

Other exploration targets will be (1) pinch-out traps of Paleogene sandstones onlapping on the ridge, and (2) relatively unknown Cretaceous reservoirs.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)