Reservoir formation and modification caused by multiple tectonic activities: Insight from calcite U-Pb geochronology in the Huangqiao area, Subei Basin

ABSTRACT The timing and origin of fluid activity are crucial for understanding the relationship between hydrocarbon accumulation and tectonic events. In this study, we conducted an integrated analysis combining petrography, geochemistry, fluid inclusion analysis, and laser ablation–sector field–inductively coupled plasma–mass spectrometry U-Pb geochronology of multiphase calcite veins collected from the Triassic Qinglong Formation carbonate reservoir in the Huangqiao area of the Subei Basin. Six phases of calcite veins were identified: C1 (141.9 ± 3.4 Ma), C2 (115.3 ± 2.8 Ma), C3 (97.03 ± 0.43 Ma), C4 (85.29 ± 0.25 Ma), C5 (circa 60 Ma), and C6 (circa 42–38 Ma). Primary oil inclusions are exclusively present in the C1, C2, and C5 veins, indicating distinct episodes of hydrocarbon migration. The fluid sources varied: C1 veins originated from hydrocarbon fluids mixed with formation water; C2 veins formed from deep hydrothermal fluids related to Tanlu fault zone activity; C3 and C4 veins derived from formation water mixed with meteoric water; C5 veins formed from intraformational water mixed with deep-seated strata water; and C6 veins sourced from deep-seated, strontium-rich crustal fluids. The ages and fluid sources of the calcite veins correspond well to key tectonic events, as well as hydrocarbon accumulation and subsequent reservoir modification accompanied by hydrocarbon leakage in the Huangqiao area. Stage I: The first phase of oil accumulation (vitrinite reflectance [Ro] = 0.7%) occurred under compressional tectonics related to the early Yanshanian orogeny. Stage II: Reservoir modification was driven by hydrothermal activity associated with the Tanlu fault zone. Stage III: Reservoir damage as late Yanshanian uplift brought the reservoir near the surface, allowing meteoric water infiltration. Stage IV: Main hydrocarbon accumulation (Ro = 0.85%) due to rapid stratigraphic burial in an extensional setting associated with Pacific plate subduction.