An improved method and indications for the compound specific isotopic analysis of hopanes in source rock extracts

Abstract

With a mixed stationary phase of 5Å molecular sieve/alumina, extracts of various kinds of source rocks are separated by column and sub-fractions dominated by hopanoid compounds were successfully obtained. The sub-fractions are suitable for the direct analysis on GC-IRMS to obtain compound specific isotopic values of hopanes. Comparison is carried out on isotopic signatures of specific hopanes in a lacustrine oil shale (Permian Lucaogou shale, Junggar Basin, NW China) and another Permian marine shale (NW of Sichuan Basin). Results show that the Permian Lucaogou shale had the δ13C values of hopanes distributed between -40.7‰ and -62.7‰, and decrease with the increasing of carbon number. The δ13C values of hopanes in marine shale of Sichuan Basin are apparently heavier, distribute between -20.5‰ and -45.4‰, and they appeared to be firstly decreased and then increased as carbon number increases. In both of the shales analyzed, the variation range of carbon isotopes of hopanes can be more than ±20‰ (±24.9‰ for the Sichuan Basin shale), which indicates multiple contribution to these compounds. The isotopic signatures of hopanes may be constrained by paleoenvironment, sources as well as maturation, however, compound specific isotope of hopanes is still a useful indicator for oil-source correlation.

Keywords

• bacteria
• hopanoid compounds
• carbon isotope
• marine source rock
• continental source rocks
• sichuan basin
• junggar basin