Evolution process and controlling factors of gas content in marine–continental transitional shales: Insights from numerical modeling

Marine-continental transitional shale (MCTS) gas holds excellent gas-generating hydrocarbon basis and exploration potential. Conducting quantitative analysis on the evolution of shale gas content and the coupled relationship between hydrocarbon generation and storage during geological history is essential for a profound understanding of shale gas enrichment mechanisms. This study establishes integrated models for hydrocarbon generation evolution, porosity evolution, and gas occurrence in Type Ⅲ organic-rich MCTS through a synergistic experimental approach combining multi-temperature methane isothermal adsorption experiments and gold-tube pyrolysis experiments on low-maturity shale samples. Simulating a variety of real and virtual burial histories and thermal histories, the evolution process of gas content in MCTS was reconstructed and the influence of various geological conditions during burial on gas content evolution was clarified. The results indicate that a seven-stage evolution (A–G) of gas content in MCTS from the Shanxi Formation, Southern North China Basin. Critical thresholds include: (1) dissolution-enhanced reservoir modification at vitrinite reflectance (EasyR_o) = 1.0%, (2) adsorbed gas saturation at EasyR_o = 1.3%, (3) dual saturation of free and adsorbed gas at EasyR_o = 2.0%, (4) 15%–30% gas loss through expulsion during overmature stages (EasyR_o > 2.0%), and (5) partial free-to-adsorbed gas conversion triggered by tectonic uplift. Total organic carbon (TOC) content and overpressure exhibit positive correlations with gas content, while tectonic uplift magnitude shows a negative impact. The influence of maximum burial depth, paleo-heat flow, and geothermal gradient demonstrate complex nonlinear relationships on gas content.