ABSTRACT

The Upper to Middle Miocene mudstones from the MITI-Hamayuchi borehole in the northern part of Hokkaido, Japan, were analyzed for steroid and hopanoid hydrocarbons in the extractable organic matter. Maturity indicators based upon their distributions increase with increasing burial depth. The correlation between sterane 20S/(R+S) ratio and R_o (vitrinite reflectance value) is similar to that reported from the Niigata and Shinjo basins, whereas the correlation between steroid TA/(MA+TA) ratio and R_o shows a different trend from that from the Niigata basin. These diverse relationships may be attributed to differences in thermal histories of the samples and kinetic properties of the indicators. The paleogeothermal gradient of this borehole estimated by the extent of epimerization of C₂₉ steranes and C₃₂ hopanes is consistent with that estimated by the observation of authigenic zeolites (Sasaki et al., 1982).