Nano-Scale Pore Structure of Marine-Continental Transitional Shale from Liulin Area, the Eastern Margin of Ordos Basin, China

Organic shale, which is deposited in marine-continental transitional environments and is found in coal-bearing strata, is well developed in China. This type of shale contains large amounts of shale gas. Here, a series of parallel experiments, including scanning electronic microscopy, low-temperature nitrogen adsorption/desorption, X-ray diffraction and geochemical analyses, were performed on 23 samples of marine-continental transitional shale from the eastern margin of the Ordos Basin in China. The results showed that the dominant minerals in the shale were quartz and clay, with average volumes of 36.31% and 52.07% respectively. The total organic carbon ranged from 0.18% to 31.16%, with a mean value of 2.43%. Based on kerogen stable carbon isotopes and maceral composition analysis, it appears that marine-continental transitional shale is characterized by gas-prone, inertinite-dominated type III kerogen. SEM images showed that the pores in the shale matrix were mainly associated with clay minerals and organic matter (OM). According to the N-2 adsorption analysis, pores of 2.5 nm and 30-50 nm in size were dominant and there were obvious differences in the mesopore (2-50 nm) volumes of the different samples. In summary, we assert that the marine-continental transitional shale pore volume comes primarily from mesopores (2-50 nm) and macropores (50 nm-300 nm) and the specific surface area comes mainly from mesopores and micropores(< 2 nm). Pore structure development is principally controlled by the type of organic matter and the mineral composition, which are different from those of marine systems. The composition of organic matter (i.e., the high inertinite content) in the study area is the main constraint on the development of organic pores. The mesopores are more developed in shales with a high content of mixed-layer illite-smectite. In addition, a higher content of brittle minerals will generate a strong rock resistant compaction ability, which helps reserve more pores.