Fluid inclusions and U-Pb dating of calcite reveal tectonic controls on differential fluid flow and alteration in fault zones

Fluid flow in fault zones is generally controlled by fault activity, yet understanding how such activity influences fluid flow processes within associated damage zones remains a significant challenge. This study reconstructs a multi-episodic fluid flow history spanning >200 m.y. within Late Triassic tight sandstone-hosted strike-slip fault zones. This reconstruction innovatively integrates U-Pb dating of calcite, fluid inclusion analysis, clumped isotope thermometry, and organic geochemistry. This study focuses on the Jinghe Oilfield in the Ordos Basin, China, where fractured reservoirs in fault damage zones host the main oil accumulations. Widely developed microfractures associated with fault activity facilitate fluid flow within the damage zones. Calcite cementation, oil charge, and secondary alteration of oil occurred successively in the fault zones. The tightness of the sandstone reservoirs primarily resulted from early calcite cementation. The accumulation of early low maturity oil followed early cementation in the fractures in the damage zones, while higher-maturity oil mainly accumulated later in sandstone pores during the late Early Cretaceous. Although regional uplift began in the Late Cretaceous, the adjustment and alteration (e.g., oil remigration and biodegradation) of previous oil accumulations in the fault zones occurred only when the faults were reactivated at ca. 5.8−3.7 Ma. Consequently, the oil retained in fractures underwent more intensive biodegradation than the oil within the sandstone pores. These results further indicate that fault-controlled fluid flow can be episodic, with transient activation events occurring during prolonged tectonic uplift (e.g., ∼100 m.y.). Oil within fault zones can still be pristine and not biodegraded unless intense fault activity occurred, at temperatures of ∼47 °C during cooling from a maximum reservoir temperature of ∼110−120 °C (corresponding to a burial depth of ∼2000 m) prior to continuous uplift.