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 δ¹³C_nC7 < δ¹³C_MCH < δ¹³C_Tol and δ²H_Tol < δ²H_nC7 < δ²H_MCH. LOCs from the eastern area have higher δ¹³C_oil, δ¹³C_Tol, and δ¹³C_MCH values than those from the western area and their δ¹³C values for C₇ hydrocarbons are similar and δ²H_nC7 < δ²H_MCH < δ²H_Tol. 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 Δδ²H_Tol–MCH versus Tol/MCH can distinguish the origins of continental facies parent materials and gas leakage of the LOCs.