This paper outlines the subsurface geoloy of Niigata Plain by describing the characteristics of several tectonic provinces from north to south. In this way the history is developed chronologically and the methods used in preparing maps of underground conditions are illustrated.
Niigata plain has recently become the most active exploration and development area in Japan. Extensive areal magnetic and advanced seismic surveys were carried out under the cooperation of several oil companies and direction of government agencies. Considerable number of exploratory wells have been drilled onshore and offshore directing the deeper and deeper formations. Comprehensive basin studies and compilation completed the preliminary investigations.
The sedimentary basin of Niigata plain has been the scene of depositional cycles. A biostratigraphic subdivision is introduced for this basin subsurface comprising Haizume, Nishiyama, Hamatsuda, Shiiya, Teradomari, Nanatani and “Lowey Tertiary” formations.
Three major Tertiary tectonic basins adjoin in northeast-southwest alignment in Niigata plain: Kita-Kambara, Nishi & Naka-Kambara and Nagaoka provinces. The general alignment of these provinces resulted from remarkable volcanisms across the generally east-west early tertiary volcanic trend of this basin.
Potential petroleum traps of following types occur: tectonic fold ings, stratigraphic pinchouts, underblock structures and synchronous highs.
Statistics, theory and field examples verify the important role of source beds for the underlying reservoirs. As the reservoir depth becomes deeper, the specific gravity of oil becomes lower. Relationship between depth and oil gravity however are different for reservoirs of each fields. The factor that cause the oil, condensate and gas variations must be related closely to lithologic character and formation-water chemistry of source beds.
