The 2003 Tokachi-oki Earthquake induced natural gas to gush out at several points in the Izumisato area of Chitose city, central Hokkaido, on September 26th. Four gas samples were taken on October 2nd and conspicuous gas seeping continued for a maximum of a month. Chemical and isotopic compositions of the gas revealed that methane was produced by CO2-reductive/hydrogenophiric methanogen and there were two different origins. Three samples had a N2-rich, CO2-depleted composition, and its methane was relatively depleted in 13C. Enrichment in N2 implies a non-marine source, but the thickness of the late Pleistocene Higashichitose formation around Izumisato is only about 20m, which is inadequate for explaining the discharged gas volume. Therefore the gas might be generated in the Higashichitose formation beneath the Ishikari lowland, and migrated upward through the strata and along the underlying uncorformity. Some gas was emitted through Izumisato fault, but the rest of the gas accumulated at a shallow depth in the eastern area from the fault, and was quickly discharged by the earthquake. Dissolution of CO2 in the formation water through these processes resulted in a low CO2 content in the gas. The other sample had a completely different compositions than the three samples. The gas was supposed to be generated in the marine Neogene Oiwake formation and migrated through or near the fault. This seepage seemed to be continuous and not related with the earthquake.
Japan hunted oil in Manchuria before World War II. In 1927 Japanese geologist found scattered asphalt in vugs of basalt and quaternary gravel deposits on the north western shore of Hulun Nur (lake), NW Manchuria. Geological Survey of China made a check survey of the area, and found the same asphalt in 1931. After establishment of Manchukuo, geological survey, tortion balance and refraction seismograph survey, and drilling were conducted during 1932-1941. One deep well (1,114 m TD) and 21 core holes were drilled on the northern shore of the lake, but no potential oil were found. That the core hole drilled in Fuxin coal mine hit oil shows in 1938, led to extensive oil exploration program. Geological field party mapped a broad anticline located on east of Fuxin, and the core hole drilled on the anticline recovered some 30 m thick sandstones (100-200 m in depth) impregnating with oil. To explore this oil called on massive drilling campaign, and 47 or more wells were drilled on this anticline, but the reservoir quality was not good enough to establish commercial production. It is noteworthy to point out that the geologists engaged in Fuxin oil exploration thought that the oil came from Jurassic shales deposited in fresh water lake. In 1940 oil shows were found in the water wells near Chengde, and a one-month field check survey was conducted, and mapped a small anticline in Jurassic shales, but no further exploration was conducted.
“Petroleum system” concept had been rapidly developed during 90's in oil industry. Magoon and Dow (1994a, b) defined “a petroleum system” as “a natural system that encompasses a pod of active source rock and all related oil and gas and which includes all the geologic elements and processes that are essential if a hydrocarbon accumulation is to exist”. Basic idea of “petroleum system” concept is to focus on fluids (oil and gas) rather than rocks. The existence of fluids (oil and gas) is essential element to have oil and gas fields in sedimentary basins, and hence accurate analysis of them is essential investigation for the increase of discovery rate in oil and gas explorations. Development of new technologies such as biomarker and basin modeling boosted the introduction of “petroleum system” concept into oil industry. Biomarker technology enables to identify effective source rocks, which supply oil and gas to tarps. Basin modeling technology reveals dynamic process of oil and gas generation, migration and accumulation, which occurred in the past.