Carbon and hydrogen isotopic compositions of C7 hydrocarbons and their geochemical significance in light oils/condensates from the Kuqa Depression, Tarim Basin, NW China

Abstract

In this study, the geochemical significance of the δ13C and δ2H values of selected C7 light hydrocarbons (nC7 = n-heptane; MCH = methylcyclohexane; Tol = toluene) is investigated in 58 light oil/condensate (LOC) samples from the Kuqa Depression of the Tarim Basin, NW China. For most of the LOCs in the western region, the nC7, MCH, and Tol exhibit systematic differences where δ13CnC7 < δ13CMCH < δ13CTol and δ2HTol < δ2HnC7 < δ2HMCH. LOCs from the eastern area have higher δ13Coil, δ13CTol, and δ13CMCH values than those from the western area and their δ13C values for C7 hydrocarbons are similar and δ2HnC7 < δ2HMCH < δ2HTol. These organic geochemical differences indicate that the LOCs from the western and eastern areas of the Kuqa Depression were mainly derived from lacustrine and coaly parent materials, respectively. The LOC reservoirs generally received a late hydrocarbon charge of the high-maturity natural gas, and the LOCs from the western Kela–Keshen areas subsequently experienced gas leakage. Hierarchical cluster analysis indicates that the most important influencing factor on the LOCs of the Kuqa Depression is the sedimentary facies (coaly and lacustrine), followed by gas leakage and then parent organic type (lacustrine sapropel and humic organic matter). A cross-plot of the Δδ2HTol–MCH versus Tol/MCH can distinguish the origins of continental facies parent materials and gas leakage of the LOCs.