Stable carbon isotope fractionation of individual light hydrocarbons in the C6–C8 range in crude oil as induced by natural evaporation: Experimental results and geological implications

Stable carbon isotope fractionation of individual light hydrocarbons in the C6–C8 range in crude oil as induced by natural evaporation: Experimental results and geological implications

The natural evaporation of a condensate from the Tarim Basin in northwest China was conducted at room temperature to investigate the stable carbon isotopic fractionation of individual light hydrocarbons (LHs) in the C6–C8 range. Our results show that individual LHs in this range in liquid residuals are progressively enriched in 13C, indicating the preferential evaporation of 12C containing compounds from the crude oil. When evaporative losses of individual LHs in the C6–C8 range are <70%, there is no significant carbon isotope fractionation (usually with δ13Cfinal–initial<1‰). A δ13Cfinal–initial of individual LHs in the C6–C8 range >1‰ mainly occurs when the evaporative losses are >70%, but the magnitudes of stable carbon isotopic fractionation strongly depend on compound classes. Compared to the straight chain and branched alkanes and the monoaromatic hydrocarbons, there is almost no stable carbon isotopic fractionation observed for cycloalkanes in the C6–C8 range. Using the Rayleigh model, stable carbon isotopic fractionation of individual LHs in the C6–C8 range during natural evaporation is described and the stable carbon isotope fractionation factors (α) are 0.998976 for n-hexane, 0.998991 for n-heptane, 0.999426 for n-octane, 0.999138 for 3-methylhexane, 0.999401 for methylcyclohexane and 0.998518 for toluene. Correspondingly, the stable carbon isotope enrichment factors (ε) are −1.024 for n-hexane, −1.009 for n-heptane, −0.574 for n-octane, −0.862 for 3-methylhexane, −0.599 for methylcyclohexane and −1.482 for toluene. Both α and ε values clearly show a progressive enrichment in 13C in the residual liquids during natural evaporation in a crude oil system (what is referred to as “normal isotope fractionation”), rather than an enrichment in 13C in the vapor phase as reported by previous studies (what is referred to as “inverse isotope fractionation”). The intermolecular binding energies due to the van der Waals attraction forces in crude oil likely plays an important role on the diffusion velocity and the escape of 13C containing LHs from the system, thereafter exerting a significant influence on stable carbon isotope fractionation, which is controlled by a kinetic isotopic effect. Since we cannot be certain of the exact amount of LHs that evaporated for one given crude oil from the subsurface to the laboratory, the results presented here strongly suggest that caution must be taken in the application of stable carbon isotopic composition of individual LHs in petroleum geochemistry and in environmental studies associated with oil pollution. However, δ13C values of cycloalkanes in the C6–C8 range can still be used in confidence to decipher oil generation, migration, accumulation and secondary alteration in the subsurface of sedimentary basins and to address oil pollution issues.

Organic Geochemistry

2012-09-01

Xiao, Qilin

Sun, Yongge

Zhang, Yongdong

Chai, Pingxia

Journal Article

QF3JBGZM

0146-6380

10.1016/j.orggeochem.2012.06.003