Origin of hydrocarbons in the Green Tuff region of Japan: 129I results from oil field brines and hot springs in the Akita and Niigata Basins

Abstract

The Akita and Niigata Basins are part of the Green Tuff region of Honshu, Japan, which hosts major occurrences of oil and gas and of hydrothermal sulfide-sulfate Kuroko Deposits. The Green Tuff formation is associated with the opening of the Japan Sea, which occurred in the Early Miocene and was accompanied by large-scale volcanic activities. We measured halogen concentrations and 129I/I ratios in oil and gas field brines and hot springs in order to determine the origin of iodine and, indirectly, of hydrocarbons in this area and their association with geological processes during the formation of the Japanese Island Arc. Based on the concentration of iodine, these samples fall into three categories: Type A (I > 190 μM), Type B (150 μM > I > 90 μM), and Type C (I < 50 μM). Samples in Types A and B come predominantly from oil/gas fields and have 129I/I ratios below 300 × 10 − 15 , corresponding to ages essentially in the Eocene epoch. In contrast, samples in Type C are generally from hot springs and have 129I/I ratios above 400 × 10 −15, corresponding to Oligocene to Miocene ages. Source formations for Type C were accumulated during the active opening of the Japan Sea, however, those for Types A and B probably are organic-rich Eocene sediments or older basement rocks which predate the volcanic formations associated with the backarc spreading of the Japanese Island. The observation that oil and gas is predominantly found in reservoir formations of middle to late Miocene age suggests mobilization of these brines in association with the tectonic and volcanic/hydrothermal activities during the opening of the Japan Sea which also led to the formation of the Kuroko Deposits in the same area. The hot spring samples of Type C reflect these processes most directly. Because of the close association of iodine with methane and other hydrocarbons, oil and gas in the Green Tuff area probably is also derived from sources considerably older than the current reservoir formations.