Controls on Variations in Hydrocarbon Phase Behavior in Ultra‐Deep Ordovician Reservoirs: Insights From Organic Geochemistry, Fluid Inclusions, and U–Pb Geochronology From the Fuman Oilfield, Tarim Basin

ABSTRACT The adjacent F I 19 and F I 20 strike‐slip fault zones in the Fuman oilfield, Tarim Basin, exhibit contrasting hydrocarbon phase behavior (volatile oil vs. condensate) in Ordovician reservoirs at comparable depths. In this study, organic geochemistry of oil and gas, fluid inclusion microthermometry, carbonate U–Pb geochronology, and solid bitumen characterization were integrated to constrain hydrocarbon sources, thermal maturity, charging history, and secondary alteration. The results indicate that condensates in the F I 20 fault zone exhibit only slightly higher aromatic maturity but markedly higher diamondoid and gas maturities than volatile oils in the southern F I 19 fault zone. Although in situ oil cracking likely occurred, invasion by oil‐cracking gas from deeper Cambrian reservoirs best explains the mixed gas origin and the anomalously elevated diamondoid and gas maturities in the F I 20 fault zone. Gas invasion is estimated to contribute >50% of the gas in the condensate reservoirs, with its contribution increasing with gas–oil ratios (GORs). Hydrocarbon phase variations are primarily controlled by the extent of gas invasion, which is governed by fault activity. Fault reactivation during the Late Yanshanian–Himalayan facilitated the upward migration of oil‐cracking gas from deeper Cambrian reservoirs into the Ordovician reservoirs of the F I 20 fault zone. Invasion of highly mature gas transformed the reservoir fluids into condensates characterized by elevated GOR and significant oil–gas maturity mismatch. Areas with evidence of Late Yanshanian–Himalayan fault reactivation and potential pathways for deep gas migration should be prioritized for condensate exploration in the Fuman oilfield. This study provides a framework for understanding hydrocarbon phase evolution in ultra‐deep fault‐controlled carbonate systems and offers a transferable approach for evaluating condensate formation in other ultra‐deep petroleum systems.