Genetic identification of light oils and condensates using δD indices of C7 hydrocarbons: A case study in the Tarim Basin, NW China

Abstract

Generally, the low concentration and homogenisation of traditional biomarkers in light oil and condensate (LOC) lead to uncertainties in their indicative significance of the parental origin of crude oils. Here, we focus on the component and stable hydrogen isotope (δD) distribution of C7 light hydrocarbons, including n-heptane (nC7), methylcyclohexane (MCH) and toluene (Tol), to identify the source rocks of LOC. In total, 114 LOCs whose source rocks are believed to be marine, lacustrine or coal facies in the Tarim Basin, NW China, are selected for the case study. The results demonstrate that the δDMCH, δDTol and δDnC7 values in the LOCs from marine, lacustrine and coal facies exhibited strong, slight and inverse ‘V' distributions, respectively, on their Line Charts. By analysing the differences among these values, we found that the ΔδDnC7–Tol and ΔδDTol–MCH values in the LOCs show good linear correlation (R2 = 0.8) and have the potential to distinguish the sedimentary facies of the source rocks of these oils. The ΔδDnC7–Tol values for marine, lacustrine and coal-formed LOCs were ˃6‰, from −20‰ to 20‰, and < −20‰, respectively. Further, the ΔδDTol–MCH values in the marine, lacustrine and coal-formed LOCs are ˂ − 5‰, from −30‰ to ‰, and > 0‰, respectively. Thus, the plot of ΔδDnC7–Tol–ΔδDTol–MCH can effectively distinguish among marine, lacustrine and coal-formed LOCs.