Hydrocarbon Generation and Corresponding Pore Evolution of Organic‐Rich Transitional Shale: A Case Study From Shanxi Formation in Ordos Basin

ABSTRACT

Transitional shales in the Carboniferous–Permian successions of the eastern Ordos Basin are critical targets for unconventional hydrocarbon exploration. However, the coupled mechanisms governing their hydrocarbon generation, diagenesis, and nanopore evolution remain poorly constrained. To address this, closed‐system thermal simulation experiments were conducted on low‐maturity Shanxi Formation shale samples (total organic carbon [TOC] = 2.87%; Ro = 0.78%) to achieve artificial maturation (Ro: 0.85%–3.86%). Finally, an integrated characterization of organic matter cracking, mineral transformation, and pore development was performed using Rock‐Eval pyrolysis, x‐ray diffraction (XRD), scanning electron microscopy (SEM), low‐temperature N ₂ adsorption (LTNA), mercury intrusion porosimetry (MIP), and nuclear magnetic resonance (NMR) analyses. The results indicate that hydrocarbon generation progresses through mature, high‐mature, and overmature stages, with expelled oil peaking at approximately ∼68.02 mg/g TOC (Ro ≈ 1.5%) and methane yields reaching 231.80 mg/g TOC during the overmature phase. Concurrent mineralogical evolution is characterized by a relative increase in quartz content alongside the degradation of clay minerals, carbonates, and pyrite, driven by the progressive transformation of kaolinite into illite, mixed‐layer illite/smectite, and chlorite. Driven by these coupled processes, reservoir porosity, pore volume (0.0027–0.0263 cm ³ /g), and specific surface area (1.097–5.309 m ² /g) exhibit a nonlinear decrease–increase–stabilization–decline trend with increasing thermal stress. Specifically, organic matter nanopores become dominant at high maturity and peak at Ro ≈ 2.7%, whereas inorganic pores are controlled by mechanical compaction, organic acid dissolution, and clay mineral diagenesis, which collectively shift the dominant pore size distributions from micropores (<5 nm) to mesopores (5–60 nm). This study establishes a comprehensive evolutionary model that explicitly links fluid generation, mineral transformation, and pore system restructuring, thereby providing a mechanistic framework for reservoir sweet‐spot prediction in transitional shales.