A practical method is used to assess maturation histories and paleotemperatures of mudstones. This method is based on 20R-C[29] sterane epimerization and the apparent heating rate. A relationship among sterane epimerization, maximum burial temperature, and heating rate is determined assuming kinetic constants of sterane epimerization. This relationship is used to reconstruct the subsidence and thermal histories of mudstones and accumulated oils in the young Nishiyama/Chuo oil field (Tertiary-Quaternary).

The estimated thickness of eroded sediments in the Haizume/Uonuma Formation, the uppermost rocks in the Nishiyama/Chuo oil field, indicate subsidence and heating rates of 2-4 km/m.y. and 80-160 degrees C/m.y., respectively, and uplift and cooling rates of 2-4 km/m.y. and 80-160 degrees C/m.y., respectively, during the late Quaternary. These rates suggest recent vigorous tectonism in the Niigata back-arc sedimentary basin. Based on geologic observations and maturation levels of crude oils, the oil generation threshold and primary migration stage correspond to degrees of sterane epimerization of 20S/(20S + 20R) = 0.20 to 0.35 and 20S/(20S + 20R) = 0.40 to 0.50, respectively. The anticlinal structure of Yoshii gas and condensate reservoir was formed prior to the late Pliocene. The therma and subsidence histories of source rocks indicate that the accumulation of hydrocarbons in Yoshii reservoir rocks started in the Pleistocene. Further maturation of these reservoir rocks and the possible addition of gases from overmature source rocks during the late Quaternary resulted in formation of the Yoshii gas-condensate reservoir.

Formation of anticlinal structure of Nishiyama reservoir began in the late Quaternary after the major stage of oil migration. Vigorous tectonism during the late Quaternary caused abrupt development of the anticline, allowing oil to accumulate to form Nishiyama reservoir.