1994 年 28 巻 1 号 p. 59-72
This paper reviews recent achievements on the origin and diagenetic alteration of hydrocarbons in oils, gases, and sediments from the Japanese fields. The chemical and carbon isotopic compositions of methane to butane have indicated biogenic origin for the hydrocarbons of natural gases accumulated in volcanic rocks. The high helium isotope ratios are not critical evidence for their abiogenic origin. Some of natural gases of dissolved-in-water type are judged to be thermogenic from their high carbon isotopic composition of methane, although they were formerly supposed to be microbial. It has been confirmed that biomarker hydrocarbons such as steranes and hopanes are effective to evaluate organic maturity and paleotemperature of sediments. Their isomeric compositions indicate the precise maturity of oils from the Green Tuff region, putting important constraint on the timing of the primary migration of petroleum. However, the organic maturity estimated by the biomarkers may be affected by factors, e.g., difference in thermal history, catalytic effect of minerals, and geochromatographic fractionation.
