Abstract
The Higashi-Niigata gas field is located in the coast to offshore area in the northern part of Niigata City, Niigata Prefecture, central Japan. The stratigraphy of the middle Pleistocene to Holocene strata is reexamined by the borehole data above 600 m depth. The strata are divided into the Shirone Formation and the Kanbara Group in descending order. The Kanbara Group is composed of alternation of conglomerate, sandstone and mudstone with diagnostic four pumice beds. These pumice beds are newly named as the Higashi-Niigata first pumice bed (HN-P1), the Higashi-Niigata second pumice bed (HN-P2), the Higashi-Niigata third pumice bed (HN-P3) and the Higashi-Niigata forth pumice bed (HN-P4) in descending order. These pumice are probably reworked sediments derived from pyroclastic flow deposits which flowed in the Agano River. The HN-P1, HN-P2, HN-P3 and HN-P4 are respectively correlated to the Numazawako pyroclastic flow about 5ka, the Tagashira tephra about 130 ka, the Sunagohara-Kubota pyroclastic flow (Sn-KB) about 220 ka, the Sunagohara-Sakasegawa pyroclastic flow (Sn-SK) about 290 ka. As the result, the upper part of the Kanbara Group including the G4-layer which is the influential natural gas layer dissolved in water is estimated with the sediment about 18 ka to 300 ka.
