New Insights Into Controlling Factors Of Pore Evolution In Organic-Rich Shale

Pore evolution in organic-rich shale is synchronically controlled by organic and inorganic diagenesis processes that are significantly different from sandstones and carbonates. Fundamental studies were conducted in characterizing pore evolution. However, pore evolution mechanisms and the main controlling factors for the particular pore size range have not been clarified yet. Herein, we conducted thermal simulation experiment, coupled with petrological, mineralogical, and organic geochemical tests on the organic-rich shales from the Permian Lucaogou Formation in Junggar Basin, China, to investigate the evolution of organic matter and mineral composition in different stages of pore evolution. For the first time, we discovered the relationship between evolution mechanisms and the particular pore size ranges. Eighteen nanometer was identified as the critical pore size boundary that connected pore evolution stages with the corresponding pore types. Pore sizes ranging in 2-18 nm, 18-50 nm, and >50 nm were separately controlled by the diagenesis process of day conversion, dissolution of nonday minerals, and thermal evolution of organic matter during the pore evolution process in the Lucaogou shale. Lucaogou shale demonstrated a three-phase pattern of pore evolution as thermal maturity increases. Porosity shows a slight decreasing trend at the first stage from low maturity to an oil window, followed by a rapid increase from the oil to gas window, and remains stable with a slight increase after entering into the high-mature stage. Different correlations are discovered between pore evolution and organic matter maturity in the particular pore size range, which sheds a light on the accurate formation evaluation in the Lucaogou shale for practical applications.