The second long-term plan period is 9 years (3 years per phase) from the fiscal year of 2012, and medium-term targets have been set for each of three phases (Phase 4 ~ 6). Phase 6 (FY 2018 to FY 2020) aims to clarify the generation process and the control method of biomethane in geological formations related to sedimentary rocks in the northern Hokkaido area, and to examine the applicability of SCG (Subsurface Cultivation and Gasification) method proposed by H-RISE and to extract the points for improvement. The SCG method was patented as “Method of Producing Methane Gas in Geological Formations from Coal and/or Diatomite included in Geological Formations (Patent 6396068)” in September 2018.
　FY 2018 was the first year of Phase 6, and the Subsurface Microbiology Research Group, Groundwater Environment Research Group, and Sedimentary Rock Research Group conducted research to achieve the above targets. The outline of researches and results in this fiscal year is as described below.

　The Subsurface Microbiology Research Group continued its research on elucidating the microbial community and its functions involved in methanogenesis in diatomaceous mudstone formations using groundwater collected from JAEA Horonobe URL. The Group conducted culture experiments with various substrate additions and found that microorganisms in groundwater collected from 140-m and 250-m underground tunnels used different methanogenic substrates. In addition, strain HJ 250, a new microorganism, was successfully obtained from groundwater collected in the past. With regard to the cultivation model, a verification experiment on methanogenesis from a solution after the reaction of lignite with hydrogen peroxide was carried out using microorganisms from Sarufutsu 25 -2 borehole groundwater instead of H-RISE microbial consortium as a research assuming in situ application of the SCG method. In comparison with the experiment using the H-RISE microbial consortium, it was found that methanogenesis is possible using microorganisms in Sarufutsu 25 -2 borehole groundwater, although it takes time to start the methanogenesis. This result provides important implications for the design of SCG in situ testing from next fiscal year. In addition, the research using humic substance decomposition microbial community which had been carried out as a part of the decomposition experiment of methane source materials in the geological formations and the research using H-RISE microbial consortium which had been used as a research on the efficient methanogenesis were integrated. As a result of attempting the methanogenesis by co-culturing these two microbial consortia, the methanogenesis was confirmed as expected. Therefore, it was verified that methanogenesis from humic substances is possible by the action of microorganisms under anaerobic conditions.
　The Groundwater Environment Research Group carried out the research for the groundwater of the multiple fields in order to clarify generation mechanism of microbial origin methane in the nature and the relation with iodine. The Group carried out the analysis by using the isotope microscope system for the calcite which filled the cracking in the coal bed of Kushiro Coal Mine, and clarified that this calcite precipitated in the environment where the microbial origin methane is produced. In Sarufutsu and Hamasato areas, the Group carried out the quantitative analysis of iodine and bromine of groundwater, rock extracted water and rock collected from each borehole, and it was indicated that iodine was in adsorption equilibrium with the geological formation components in present geological formations. In addition, in order to clarify the effect of the electron donating ability of coal and the existence of iron (Fe) with the generation potential of hydroxyl radical on the reactivity of coal with hydrogen peroxide, a reaction test of phenol and the like with hydrogen peroxide, and a reaction test of reduced electron donating ability lignite and reduced Fe content lignite with hydrogen peroxide were carried out. These results suggest that the hydroxyl radical generated by the reaction of Fe with hydrogen peroxide functions as a radical initiator, and that the decomposition of coal itself proceeds through the radical chain reaction of the organic framework. Furthermore, chemical morphological analysis of Fe suggested that Fe in coal was mainly adsorbed on humic acids and existed in the form of pyrite and iron sulfate.
　The Sedimentary Rock Research Group conducted the following studies in preparation for SCG in situ testing from next fiscal year. First, the Group conducted field tests on a high-precision water level observation method to evaluate the permeability and its change of in situ lignite formations. As a result of carrying out continuous measurement by installing a throwing type pressure gage at about 50-m depth of existing Sarufutsu 25 -1 borehole, there was a fluctuation in the pressure gauge output. However, it was confirmed that there was almost no water level fluctuation, when it was corrected considering the atmospheric pressure data. As a result, high-precision water level observation in the SCG in situ testing became possible. In addition, in the SCG in situ testing, since the gas generated in hydrogen peroxide injection into lignite formations may reduce the permeability, indoor experiments were conducted to understand this phenomenon. For the purpose of the establishment of indoor test methods and the system construction, the injection test of pure water and high-dense hydrogen peroxide aqueous solution was carried out using bituminous coal in this fiscal year. Concretely, in order to reproduce in situ cracking, the fracture surface was formed by controlling the circumferential displacement of the cylindrical specimen, and the injection test was carried out in this condition. As a result, when an aqueous solution containing 5% hydrogen peroxide was injected, permeability was observed to decrease by more than two orders of magnitude compared with pure water, and it was confirmed that control of aqueous solution concentration was important in the injection of hydrogen peroxide aqueous solution. Though the test was carried out under the limited condition in this fiscal year, it is considered that indoor test methods of hydrogen peroxide injection to coal and the system were established.