ABSTRACT Gulong shale oil in the first member of the Qingshankou Formation, Songliao Basin, has considerable exploration potential, yet the evolution of its reservoir remains poorly constrained. Clarifying this evolution is essential for evaluating reservoir properties and identifying prospective intervals. In this study, Gulong shale core samples from the first member of the Qingshankou Formation were collected from multiple wells across a maturity range from immature to highly mature. Pore evolution was investigated using N 2 adsorption, high‐pressure mercury intrusion (MICP), and argon ion beam milling scanning electron microscopy. Results reveal that porosity evolves non‐monotonically, decreasing from the immature to mature stages, increasing at the highly mature stage, and then slightly declining at the highest maturity. Concomitant with this evolution is a shift in the main control factors, with inorganic pores governing reservoir characteristics at immature–mature stages and organic pores becoming the primary control at high maturity. Overall, thermal maturation, diagenesis, and hydrocarbon generation and expulsion processes jointly govern reservoir quality, suggesting that, under favorable preservation and pressure conditions, highly mature shale may hold exploration potential.