Dynamic evolution of shale oil occurrence in lacustrine shales: Lithological constraints and implications for in-situ conversion

The occurrence state of shale oil is crucial for its mobility and development, which is closely linked to petroleum generation and pore system evolution; however, the dynamic coupling between them across different lithologies remains poorly constrained. Based on the results of simulation study on calcareous, felsic, and argillaceous shales under water-saturated conditions, this study reconstructed the evolutionary pathway of shale oil occurrence from the early oil generation stage to the light oil stage. The results suggest that pore structure evolution in lacustrine shales exhibits distinct lithological dependence. With increasing maturity, the pore volume and specific surface area of felsic and calcareous shales significantly increase, primarily due to the development of dissolution pores and organic matter-related pores, while argillaceous shale displays weaker growth. Furthermore, oil occurrence in lacustrine shales is jointly controlled by petroleum generation and pore structure, which undergo complex and dynamic evolution with increasing maturity. At the main oil generation stage, shale oil occurs in both mesopores and macropores, with its distribution significantly governed by the mesopore proportion. The oil saturation in mesopores and macropores is negatively correlated with the fractal dimension D 1 , suggesting that a more complex pore structure tends to form a lower oil saturation. However, the constraints of pore structure on shale oil occurrence significantly decrease during the light oil stage, because light hydrocarbon greatly enhances oil mobility. Based on laboratory results, the influence of lithological and thermal maturity on lacustrine shale oil development and in-situ conversion has been discussed. For calcareous and felsic shales, the significantly high meso- and macropore volumes during the main oil generation stage result in good reservoir performance and high productivity at moderate maturity (EqV R o = 0.8%–1.1%). In contrast, a higher maturity stage (EqV R o > 1.0%) is required during in-situ conversion for argillaceous shale due to the slow pore growth and strong adsorption capacity. These findings have important implications for shale oil enrichment and support the optimization of exploration and in-situ conversion strategies in lacustrine shales.