Multiscale pore structure characteristics of deep marine shale and its control on the gas transport mode

Deep marine shale has undergone intricate geological evolution, with pore development at the nanoscale. Consequently, quantifying the impact of the pore structure on gas is challenging. In this paper, the microscopic space structure of deep shale was quantified, and the correlation between the pore structure and the mode of gas transport was established. This study focused on the Silurian Longmaxi Formation shale, using experimental testing and fractal dimension methods to quantify pore characteristics and control factors at multiscale, and establishing the relationship between pore structure and gas transport mechanism. The results indicated that the main lithofacies are organic-rich siliceous shale and organic-rich mixed shale. The preservation of pores by siliceous minerals and high pressure is the key to the high-quality reservoir. Based on gas transport mechanisms, there are five types of gas transport modes: surface adsorption-diffusion, Knudsen diffusion, Fick diffusion, slip flow, and continuous flow. The critical pore size was 1.2, 4.34, 15, and 180 nm, respectively. Compared to shallow shale, the abundance of micropores and mesopores in deep shale is greater, and the contribution of microfractures to shallow shale gas transport is crucial. In conclusion, favorable exploration in targeting organic-rich siliceous shale with low D1 and high D2, as well as organic-rich mixed shale segments with high D1 and high D2. Specifically, within hydrocarbon-rich basins located below 3500 m, it is recommended to search for overpressure regions with weak structural deformation. This research establishes a basis for the exploration and geological principles of deep marine shale.