Abstract
Crude oils and condensates from the Japanese Neogene Tertiary sedimentary basins were analyzed for steroid and triterpenoid hydrocarbons, using GC/MS. Most crude oils show incomplete side chain epimerization of C29 steranes. This is possibly due to an insufficient reaction time and temperature for epimerization since the Miocene age. All oils show a good correlation between nuclear isomerization and side chain epimerization of C29 steranes. The correlation curve appears to be different from that previously reported, which may have resulted from a difference in thermal histories. On the basis of the extents of side chain cleavages of saturated and triaromatic steroid hydrocarbons as well as upon the extents of configurational isomerizations of steranes and hopanes, the condensates are considered to be distinctly maturer than the crude oils. This fact suggests that the condensates have undergone higher temperatures than the crude oils. Among the crude oils, those from the Niigata basin are significantly maturer than those from the Akita-Yamagata basin. This is probably due to the difference between the oil source rocks in the two basins, which may cause a difference in the stage of primary migration of oils. The conversion of monoaromatic to triaromatic steroid hydrocarbons does not correlate well with the side chain epimerization of steranes. This may be due to preferred migration of monoaromatics by geochromatographic separation. There is a significant difference between the crude oils from the Niigata basin and those from the Akita-Yamagata basin in respect of the C27/C29 ratios of steroid hydrocarbons. This difference is consistent with the previous report on visual kerogen compositions of oil source rocks from these basins. The relative abundance of C27 to C29 of monoaromatic steroid hydrocarbons is thought to be less affected by thermal stress than that of steranes. This could thus be a more useful organic source indicator in the case off matured oils such as condensates in Japanese Tertiary sedimentary basins.
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