Characteristics of and controlling factors for pore water occurrence in deep marine shales in the Qiongzhusi Formation, Sichuan Basin

The shale reservoirs of the Lower Cambrian Qiongzhusi Formation in the Sichuan Basin exhibit complex pore structures, which provide important spaces for pore water occurrence. However, the specific occurrence characteristics and their controlling factors remain poorly understood. In this study, we investigate deep marine shales from the Qiongzhusi Formation in the basin. By integrating a series of experiments, including water saturation centrifugation-nuclear magnetic resonance (NMR) combined tests, scanning electron microscopy (SEM), nitrogen adsorption, and molecular dynamics simulation, we systematically reveal the microscopic occurrence characteristics of pore water in the shale reservoirs and their primary controlling factors. Accordingly, the corresponding pore water occurrence patterns are established. The results indicate that in deep marine shale samples from the Qiongzhusi Formation, totally irreducible fluids occur primarily in pores with a diameter smaller than 4. 98 nm. In contrast, partly movable fluids are distributed in pores with sizes ranging from 4. 98 nm to 8. 54 nm, while pores larger than 8. 54 nm mainly contain totally movable fluids. Molecular dynamics simulations reveal that water molecules are preferentially adsorbed on the surface of inorganic minerals, followed by methane molecules. With the increase of temperature and pressure (corresponding to the increase of buried depth of strata), the adsorption capacity of both increases, but the presence of water molecules has a significant inhibitory effect on methane adsorption. Under high water saturation, the proportion of movable water increases significantly, while tectonism-induced microfractures further enlarge its occurrence space. Concurrently, the increased proportion of large pores reduces capillary pressure, leading to a decreased proportion of irreducible water which tend to primarily concentrate in small pores. Mineral composition, pore structure, tectonism, and the water saturation of reservoirs jointly constitute an interconnected system that governs the occurrence state of pore water in shales of the Qiongzhusi Formation. These findings serve to lay an important basis for advancing geological theories on deep shale gas accumulation.