Fine characterization of lithofacies and pore network structure of continental shale: Case study of the Shuinan Formation in the north Jiaolai Basin, China

There is a continuously growing collection of research on continental shale reservoirs. This paper characterizes the Lower Cretaceous Shuinan Formation, a continental shale in the Jiaolai Basin, China, by adopting the improved ternary diagram of carbonate and clay minerals, and argon ion milling-SEM, high-pressure mercury injection, low-temperature N-2 adsorption, low-temperature CO2 adsorption and related geochemical experiments. The pore network structure is visually characterized under an electron microscope and indirect quantitative statistics of reservoir space. The influence factors of the full-scale pore development are classified according to the geochemistry and mineral content of different lithofacies within the shale reservoir. The results reveal that, there are five types of shale lithofacies in the Shuinan Formation: siliceous calcareous shale lithofacies (Ca1), calcareous/siliceous mixed shale lithofacies (F2), calcareous siliceous shale lithofacies (Si3), mixed siliceous shale lithofacies (Si2) and argillaceous siliceous shale lithofacies (Si1). The pores are characterized as intergranular pores and intragranular pores, with microcracks dominating the pores. Combined with the results of surface porosity and joint characterization of the pores, the pores within the shale in the Shuinan Formation that would be advantageous for shale gas development are mainly mesopores and macropores, greater than 10 nm in diameter. Pore development is controlled by mineralogy. Brittle minerals are beneficial to the development of micropores, mesopores and macropores, while carbonate minerals are not conducive to the development of pores in general. Filling occurs mainly in pores of 30-50 nm range. Finally, based upon the lithofacies classification, geochemical properties and pore characterization, the most suitable sections of the LC-1 well for shale gas development are concentration in the middle-lower Shuinan Formation.