Abstract
Biomarker compositions were examined for bitumen extracts of two borehole sequences from the Niigata sedimentary basin by GC/MS. Maturity parameters based on sterane and triterpane isomeric compositions revealed that the Teradomari Formation of both sequences has not reached the primary zone of oil formation, yet.
The maturity parameters reversely correlate with depth in the upper part of immature zones. Highly matured isomeric compositions in shallower samples might result from addition of reworked biomarkers or production of thermodynamically stable isomers, such as 20S, α β β and α β configurations, by double bond isomerizations of sterenes and triterpenes, and subsequent reduction. The reversal maturation trend is explained by the supply of more matured biomarkers reworked from Neogene sediments, successively uplifted and eroded at the margin of the basin. Alternative explanation is dilution of the matured biomarkers with products from free, or kerogen- and/or asphaltene-bound precursors with biological configurations, such as ααα 20R-steroids and ββ 22R-triterpenoids, through early diagenesis.
