Three oil samples, eight gas samples, and three rock samples, collected mainly from surface seepages in the northern Fossa Magna region, Nagano Prefecture, Japan, were subjected to geochemical analysis, yielding the following results. 1) The oils are heavy and biodegraded, and are classified as Kubiki-type (Hirai et al., 1995). Geochemical characteristics of the oils indicate an origin from type II-III organic matter, and their maturity is estimated to be ∼ 0.7% Ro equivalent. 2) The gases are mainly methane (C1); only three samples from Tenjinzawa and Nakatsuka contain heavier hydrocarbons, up to C6, and these samples have with high wetness over 10%. The chemical and isotopic compositions of the gases indicate that they are partly biodegraded and mostly thermogenic in origin, with a maturity of 0.7%-0.9% Ro equivalent. 3) Mudstones in the Bessho Formation and the lower part of the Aoki Formation are probably the source rocks that generated the oil and gas in the areas of the Takafu and Komeji synclines as indicated by source rock analysis and by surface oil and gas seepages. Although subsurface geologic structures remain unclear, there is hydrocarbon potential in the zone between the areas of oil and gas seepages and the synclines.
The producing reservoirs of the Ojiya oil field, Niigata Prefecture, which belong to the Uonuma Group in the northern part (Yamaya area) and the Haizume Formation in the southern part (from Jigokudani to Kirikubo areas), are correlated to the Nishiyama stage in the Niigata well stratigraphy. Head space gases from the MITI Oguni exploratory well and surface seepage gases are mainly thermogenic origin based on their carbon isotopes, whose maturity is estimated to be 1.0 to 1.1 % Ro equivalent, and were probably generated in the Lower Teradomari Formation below 4,000 m in depth. As oil properties such as specific gravity and distillation test result are clearly different between the Yamaya oil and the others (the southern oil), the Yamaya pool is separated from the others. The southern oil geochemically resembles the Sagara oil, Shizuoka Prefecture, which indicates that the oil were generated in the deeper part of the basin, migrated upward through faults, laterally migrated passing through the gas showing intervals in the MITI Oguni well, and accumulated. The Yamaya oil was probably migrated through the reverse fault in the Nishiyama Formation of the Ojiya SK-1 and SK-2 wells.
Over the past decade, the deep subseafloor biosphere has been explored systematically through scientific ocean drilling, revealing that numerous microbial cells are present in dark and old sediments down to over 1,000 meters below seafloor. The subseafloor microbial ecosystem plays significant ecological roles in biogeochemical carbon cycle, including degradation of organic matter to methane and formation of hydrocarbon reservoirs. The nature of subseafloor microbial ecosystem provides us some clues to address significant societal issues; e.g., to reduce CO2 emissions and increase energy efficiency. To discuss the present status and further study potential of these research fields, “The symposium for subseafloor hydrocarbon resources and carbon cycle: Geobiotechnology for sustainable earth and societal systems” was held on January 24th, 2014 in Koshiba Hall, the University of Tokyo. In this public symposium, the recent studies of subseafloor life and carbon cycle were reviewed, and then possible engineering and/or technological applications of bio-renewable CO2 and other useful functioning were discussed for developing sustainable carbon and energy circulation systems in the future.