Abstract
To clarify the timing of primary migration of petroleum hydrocarbons in the Japanese Tertiary Niigata Sedimentary Basin, maturation levels of some crude oils and condensates were evalutaed based upon biomarker compositions (steranes and hopanes). Consequently, it was revealed that most oils were at the similar organic maturation levels. This maturation level can be represented by 0.45 to 0.50 of the degree of sterane epimerization (20S/(20S+20R) ratio of C29-sterane) and about 0.3 of the degree of sterane cracking(L/(L+H)ratio=(C21+C22)/(C21+C22+C27+C28+C29) ratio of five 5α-steranes). This can be also correlated to the 0.6 to 0.7 of Ro% (vitrinite reflectance) in the Niigata Sedimentary Basin. This indicates that the primary migration of petroleum hydrocarbons from mudstone source rocks started prior to this maturation stage. Considering that the generation of petroleum hydrocarbons starts at about Ro%=0.5 in the Niigata Sedimentary Basin, primary migration most probably occurs at Ro%=0.5 to 0.7.
There is a marked difference among L/(L+H) ratios of condensates in which the sterane epimerization is nearly at the end point. The different L/(L+H) ratio of each condensate oil (0.48 to 1.00) shows a difference in maturation level. The L/(L+H) ratio of steranes can be applied to evaluate the organic maturation at a relatively higher stage. The L/(L+H) ratio is also useful as a parameter for the evaluation of thermal cracking of C-C bond in source rock.
