Thermally Induced Diagenesis and Pore Space Evolution of Trona-Nahcolite Aggregates in Continental Alkaline Lacustrine Shale Oil Reservoirs

Continental alkaline lacustrine shale oil reservoirs are typically characterized by extensive fracture networks infilled with alkaline minerals, such as trona and nahcolite. These mineral veins play a crucial role in hydrocarbon storage and migration. However, the diagenetic evolution of these minerals during thermal maturation and their impacts on reservoir storage capacity remain inadequately understood. In this study, thermal simulation (pyrolysis) coupled with integrated mineralogical characterization techniques (including XRD, SEM, TG-DSC, XPS, and FTIR spectroscopy) was systematically employed to investigate the phase transformation and pore structure evolution of alkaline fracture-fillings. Results indicate that a unique synergistic thermal instability exists within the trona-nahcolite assemblage. Specifically, the in-situ dehydration of trona releases structural water, which creates a localized hydrothermal environment and significantly facilitates the decomposition of coexisting nahcolite. Concurrent with these transformations, a substantial solid volume reduction (~38%) is induced. Consequently, the initially dense mineral veins are converted into porous frameworks, leading to a significant expansion of pore space. Thus, we propose that this thermally driven mineral conversion serves as a key diagenetic mechanism for secondary porosity generation. It is concluded that this phenomenon significantly contributes to the formation of effective reservoirs in deep alkaline lacustrine basins, thereby providing novel insights for the evaluation of continental shale oil resources.