Micropaleontology has been used for both the determination of geological age and correlation to the Neogene-Quaternary formations in the oil fields in Japan. However, as it is against the rule of International stratigraphic guide to correlate to standard formations based on micropaleontology, it sometimes causes confusion of the stratigraphy of the oil fields. Micropaleontology is a useful tool for not only the correlation to the geological age but also the paleoenvironment analysis. The ecology of calcareous nannoplankton is also applicable to reconstruct the stability of the surface ocean condition related to nutrient supply during the Miocene to Pleistocene. I describe in detail the characteristics of the microfossils both for the determination of geological age and for paleoenvironmental analysis, and introduce the utility of micropaleontology for petroleum exploration in this papaer.
Pore waters collected from the eastern margin of the Japan Sea including gas hydrate fields off Joetsu were analyzed for concentrations of major and minor dissolved species in order to characterize fluid migration in high methane potential area. Rapid decreases of sulfate concentration and accompanying increases of alkalinity and hydrogen sulfide concentrations point to active diagenesis of organic matter such as anaerobic methane oxidation at the sulfate-methane interface, indicating high methane flux and subsequent gas hydrate formation near the seafloor on the Umitaka Spur and Joetsu Knoll. Gas hydrate occurrences are inferred generally by ion dilution and enrichment in 18O and deuterium of pore waters, ion enrichment and depletion in 18O and deuterium, to the contrary, indicate rapid growth of massive gas hydrate exceeding ion/water diffusion. Alkalinity increase also results in the precipitation of carbonates near the seafloor, corresponding to rapid decrease of Ca concentration. Pore water geochemistry changes remarkably in the shallow interval, induced mainly by active methane flux to the water column. Concentration of Cl and isotopic compositions of oxygen and hydrogen reflect inputs of deep fluids as well, deep-sourced material can be delivered to the shallow interval. Long-termed fluid migration is also evident in the research area.
We investigated molecular and stable isotope compositions of dissolved gas in pore water in subsurface sediment cores that were retrieved from the eastern margin of Japan Sea during the MD179 cruise onboard R/V Marion Dufresne in June 2010. Hydrate-bearing sediment cores retrieved from Umitaka Spur in the Joetsu Basin showed high 13C concentrations of methane, indicating its thermogenic origin, whereas those at Joetsu Knoll partly contained microbial methane because 13C and deuterium are both depleted. Other sediment cores without gas hydrates showed mainly microbial methane formed via CO2 reduction. The profiles of methane and CO2 in the sediments showed a minimum concentration of 13C at SMI (sulfate-methane interface) depth. The concentration of methane in the sediments increased dramatically beneath the shallow SMI depth in the Joetsu Basin, indicating active microbial methane generation.
Gas hydrates are attracting attention as a next-generation energy source. On the other hands, Methane gas contained in the natural gas hydrates has approximately 20 times the greenhouse effect of CO2. There are concerns that dissociation of methane gas from the gas hydrates distributed in submarine surface layers (shallow type gas hydrates) by rising ocean temperatures or by vaporization at recovery of the hydrates for energy may contribute to global warming, which in turn may raise the sea level and causing climatic instability. In addition, many gas-hydrate-bearing areas are distributed near the boundaries of tectonic plates, as shown in the figure. Gas hydrates in the surface layer of seafloor may dissociate when seismic activities cause seafloor landslides that in turn cause gas hydrate-bearing layers to fail. There have been concerns over the environmental effects of shallow type gas hydrates, but surveys for shallow type hydrates have been few. The purpose of this study is to understand the soil properties of the sea-bottom sediment obtained from the Eastern Margin of Japan Sea. The samples were collected from 5 regions (Umitaka Spur, Joetsu Ridge, Toyama Trough, West of Tsugaru and west of Okushiri Ridge), and the physical (soil density and water content) and mechanical (cone penetration ratio and vane shear strength) tests were performed by on-board used for the sea-bottom sediments. From the test results, it is found that the physical properties of the sediment samples obtained in these areas have no marked difference regardless of the locations or depth of sampling. However, the soil strength of sea-bottom sediments obtained gas hydrates was lower than those of other sediments. It would seem that this is because the effect of methane hydrates dissociation and vaporization of dissolved gas in the pore water at sampling.