Large-scale CO2 accumulation and oil cracking associated with subduction and Tan-Lu Fault Zone strike-slip in the Huangqiao area, Subei Basin, East China

Mantle-derived CO2 fluids are widely distributed along the Tan-Lu Fault Zone in eastern China, yet their roles in petroleum system evolution remain poorly constrained. This study investigates the timing of mantle-derived CO2 fluids and oil charging, reservoir pressure evolution, and hydrocarbon alteration in the Huangqiao area of the Subei Basin, integrating fluid inclusion petrography and microthermometry, Raman spectroscopy, burial and thermal history reconstruction, and reservoir pressure estimation. Aqueous fluid inclusions coeval with two-phase (liquid oil + vapor) oil inclusions, three-phase (liquid oil + vapor + solid bitumen) oil inclusions, and CH4 gas inclusions yield homogenization temperatures (Th) of 114.2–136.2 °C, 89.5–116.0 °C, and 140.4–152.2 °C, respectively. These temperatures indicate two oil charging events, at ca. 151 Ma and ca. 63 Ma, and one CH4 charging event at ca. 35 Ma. In contrast, aqueous inclusions coeval with CO2-rich inclusions and mixed gas inclusions yield significantly higher Th values of 156.1–197.2 °C (average 168.1 °C), exceeding the Longtan Formation’s (P2l) modeled maximum burial paleotemperature (~150 °C). Raman spectroscopy of solid bitumen yields equivalent vitrinite reflectance values (VReq) of ~1.88%–2.93%, markedly higher than maturity levels predicted by burial heating alone. Reservoir pressure reconstruction further reveals pronounced overpressure from CO2 charging (maximum pressure coefficient ~1.90), indicative of large-scale CO2 charging in the Huangqiao area. Integration of thermal, pressure, and fluid inclusion evidence indicates that crude oil in the Huangqiao area experienced significant thermal alteration and oil cracking-related transformation, which cannot be explained solely by short-lived burial heating (~150 °C). Instead, intense thermal input from large-scale mantle-derived CO2 charging, linked to 64.5–56.1 Ma volcanism driven by Pacific Plate subduction and Tan-Lu Fault strike-slip, is consistent with the available evidence and may have significantly contributed to the observed hydrocarbon alteration. Thermogenic CH4 generated from oil cracking was trapped under near-hydrostatic conditions during tectonic adjustment associated with the Sanduo orogeny. This dynamic balance between heat supply, pressure evolution, and fault activity explains the coexistence of light oils, condensates, CO2, and thermogenic CH4 in the Huangqiao area.