Abstract
The writers have studied the generation and migration of oil and gas in Japan from the geochemical viewpoint and the results are summarized as follows: In the Niigata gas fields, methane is recognized in the whole geothermal horizons; ethane is only found in those over 40℃ and propane is over 60℃. On the other hand, the results of geochemical surveys, such as CPI of normal paraffin wax, hydrocarbonization index and distribution of gaseous hydrocarbons for several deep stratigraphic wells drilled in the oil productive areas of Japan, suggest the principal oil generation has occurred during diagenetic to catagenetic stages. Therefore, both early and late oil generation hypotheses may be applied for the Neogene formations in Japan. From the analyses of perylene, Ni-porphyrin and VO-porphyrin in muddy core and crude oil samples of the Mitsuke oil field in Niigata prefecture, downward migration for the oil in upper reservoirs and lateral and upward for lower reservoirs are concluded. The weight ratios of hydrocarbons in reservoirs to source rocks in several Japanese oil fields ranges from 1/3 to 1/15. These values are much higher than those in the United States. As the weight ratios of estimated oil in reservoirs to expelled interstitial water are much more higher than the solubility of liquid hydrocarbons in water, oil had not been migrated from source rocks to reservoirs in the form of true solution.
