Improved light hydrocarbons collection method for the pyrolysis of crude oil in gold tube closed system experiments

Abstract

Light hydrocarbons (C6–C13) are the major components of light oils and condensates and contain abundant geochemical information. Hence, they are widely used as important supplements to biomarker analysis. Due to high volatility, it is often difficult to collect light hydrocarbons completely after pyrolysis experiments. The traditional liquid hydrocarbon (TLH) collection method is usually applied to collect liquid hydrocarbon products after gold tube closed-system pyrolysis experiments. However, this method is difficult to adequately inhibit the evaporation of light hydrocarbons, which may cause deviations in the evaluation of generation potential, applications of parameters, and stable carbon isotopic compositions of light hydrocarbons. Hence, we have developed an alternative method of liquid nitrogen direct freezing (LNDF) to collect liquid hydrocarbon products after pyrolysis of a crude oil in closed gold tubes. The yields, molecular and stable carbon isotopic compositions of light hydrocarbons collected are compared with the TLH method to evaluate any differences. In addition, the influence of thermal alteration on the characteristics of light hydrocarbons are discussed. The yields of light hydrocarbons by the TLH method are 10–75% less than that by the LNDF method, which may bring about an underestimate of light hydrocarbon generation potential. The maturity corresponding to the maximum light hydrocarbon yield determined by the TLH method is at Easy%Ro = 1.08, much lower than Easy%Ro = 1.67 by the LNDF method. Relative to the LNDF method, the TLH method reveals significant differences in some light hydrocarbon parameters and C6–C8 carbon isotopes, which may cause deviations in their geochemical applications such as oils correlation and thermal maturity evaluation. In addition, thermal maturation has significant effects on some light hydrocarbon parameters. The LNDF method can effectively avoid evaporative loss to reveal the original geochemical information of light hydrocarbons.