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ABSTRACT: Deposition of source and reservoir rocks and formation of trap structures in response to basin inversion of rift basins in Niigata and Akita oil fields, Japan

Takano, Osamu , Technology Research Center, Japan National Oil Corporation, Chiba, Japan

Most of oil and gas fields in Japan are concentrated in the Neogene Niigata and Akita Basins, located along the eastern margin of the Sea of Japan. The Niigata and Akita Basins were generated as rift basins related to the opening of the Sea of Japan around 16Ma, and were inverted into compressional basins around 6.5Ma due to a change of plate tectonic condition. This basin history brought a scenario to make a good combination not only between source and reservoir rocks, but also between reservoir rocks and trap structures. This paper presents the relationship between tectonic basin history and formation of source, reservoir rocks and trap structures in a rifted and inverted basin.

The basin history is divided into four tectonostratigraphic stages. Stage I (16Ma-13.5Ma) is a syn-rift to early post rift stage, showing rapid basin subsidence and deposition of slope to basin floor shale. Stage II (13.5Ma-6.5Ma) is a post rift stage, showing exponential decrease of subsidence rate. Good source rocks, which consist of siliceous shale, were deposited largely, although submarine fan turbidites, which are good for reservoir rocks, were dominated in the southern part. Stage III and IV (6.5Ma-Present) are a compressional stress field stage after the basin inversion. Due to uplift of provenance areas, coarse clastics were supplied and submarine fan turbidites were deposited widely, which resulted in good reservoir rocks. Compressional stress made syn- and post depositional folds and faults, which formed anticlinal trap structures during this stage.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia