Differences in the distribution and occurrence phases of pore water in various nanopores of marine-terrestrial transitional shales in the Yangquan area of the northeast Qinshui Basin, China

Shale gas reservoirs generally contain pore water that affects their gas-bearing properties; however, the differences in the water-bearing characteristics of various shale nanopores remain unclear, especially for marine-terrestrial transitional shales. This study collected a set of fresh shale core samples from the marine-terrestrial transitional strata of the Qinshui Basin and investigated the distribution and occurrence phase of pore water in nanopores with diverse pore widths (micropores and non-micropores) and pore types (organic-matter hosted (OM) and inorganic-matter hosted (IM) pores). The results indicate that an average of 12.98% of the pore water is stored in the OM pores and featured by monolayer adsorption on the OM micropore surfaces, while the OM non-micropores are free of pore water. An average of 87.02% of the pore water is stored in the IM pores, 47.01% and 37.17% of the pore water exhibits monomolecular adsorption on the surfaces of IM micropores and non-micropores, respectively, and the rest of the pore water adsorbs on the two types of IM pore surfaces forming double-molecular or multiple-molecular layers. The pore water takes up the majority of the IM micropore structures and the IM non-micropore surfaces; however, it only occupies a few of the OM micropore structures and fails to occupy the OM non-micropore structures. For the studied shales under as-received conditions, 39.24% of the pore surface areas and 65.99% of the pore volumes are effective for storing shale gas. The effective pore surfaces are predominantly determined by the OM pores, while the effective pore volumes are contributed equivalently from the IM and OM pores. Therefore, the adsorbed gas of the shales is mainly stored in the OM pores, while the IM pores may only contain half of the total free gas.