Very Low‐Grade Metamorphism and Paleogeothermal Recovery of the Upper Palaeozoic in the Suhongtu Depression, Yin‐E Basin, North China: Constraints From Illite, Vitrinite Reflectance, and Fluid Inclusions

The Yin‐E Basin, located at the junction of the Siberian, Kazakhstan, and Tarim blocks and the North China Craton, has experienced complex tectonic activities and remains one of the underexplored onshore sedimentary basins in China. The Upper Palaeozoic is an important stratigraphic interval for oil and gas exploration, but its source rock thermal evolution lacks systematic research, thus hindering exploration progress. Addressing the frontier topic of very low‐grade metamorphism's role in organic maturation, we studied the clay mineralogy (illite crystallinity: 0.42°–0.25° Δ2 θ ), illite polymorphism (predominantly 2 M1), and cell parameters (b 0 : 9.0024–9.0204 Å) of the Upper Palaeozoic source rocks (wells YBC1, BD1 and YBN1) in the Suhongtu Depression, revealing the palaeogeothermal field of the Upper Palaeozoic. These data were combined with basin modelling to quantitatively constrain the thermal evolution history. The results indicate that the Upper Palaeozoic strata primarily underwent prehnite‐pumpellyite‐facies of very low‐grade metamorphism under medium‐low pressure, corresponding to peak temperatures of 211.94°C–226.32°C. The reconstructed palaeotemperature reached 210°C–220°C. By the end of the Permian, all source rocks had reached their maximum thermal maturity (vitrinite reflectance, R o: 1.42%–2.42%), with the Ba'nan Sag showing significantly higher maturity ( R o: 1.57%–2.42%). This study provides key constraints on the thermal evolution and hydrocarbon generation potential of Upper Palaeozoic source rocks, supporting future exploration in the Yin‐E Basin and adjacent areas.