Thermal evolution of light hydrocarbon fingerprints in biodegraded oils from Ordovician reservoirs, Tabei Uplift,...

Within the Tabei Uplift of the Tarim Basin, Ordovician reservoirs in both the northern Halahatang (N-Halahatang) and western Lunnan (W-Lunnan) areas experienced extensive biodegradation during the Late Hercynian (Permian). Subsequent Himalayan (Neogene-Quaternary) tectonism induced divergent burial-thermal histories: the N-Halahatang reservoirs underwent intensive maturation (> 6,500 m depth; 1.02-1.22% R_o), while the W-Lunnan reservoirs experienced milder maturation (< 5,800 m depth; 0.70-0.85% R_o). Despite similar δ¹³C_oil values indicating genetic affinity, the relatively deeply buried biodegraded oils from the N-Halahatang area contain abundant C₆-C₈ light hydrocarbons (LHs), while the biodegraded oils from the W-Lunnan area exhibit only trace amounts of C₆-C₈ LHs. To elucidate the evolution of LHs compositions and fingerprints in biodegraded oils under thermal maturation, and to determine whether the more enriched C₆-C₈ LHs in the N-Halahatang oils can be attributed to enhanced burial-thermal maturation, two relatively shallower-burial biodegraded oils (Well LG40: slight to moderate biodegradation‌; Well LG7: heavy to severe biodegradation) from the W-Lunnan area were artificially pyrolyzed to various maturities. Subsequently, LH parameters of the pyrolyzed oils were compared with those of the naturally matured, deeply buried oils (heavy to severe biodegradation) from the N-Halahatang area. The results indicated that both biodegraded oils generated C₆-C₈ LHs through thermal cracking, and the more severely biodegraded oil (Well LG7) exhibited a lower LH maximum yield than that from Well LG40. Certain parameters for organic matter type classification (n-C₇-DMCP-MCH and 3RP-5RP-6RP diagrams) generally remained applicable during thermal maturation, whereas most parameters for secondary alteration identification and maturity assessment were significantly compromised. Additionally, LH parameters of the N-Halahatang oils (1.02-1.22% R_o) matched those of the LG7 pyrolyzed oils at EasyR_o = 1.00-1.20%, confirming that the enriched C₆-C₈ LHs in the N-Halahatang oils can be attributed to cracking of biodegraded oils (with ‌biodegradation levels equivalent to Well LG7‌) under intense burial-thermal maturation. Furthermore, the potential C₆-C₁₃ LHs derived from biodegraded oil cracking constitute 11-16 wt% of N-Halahatang's liquid hydrocarbon resources.