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 clay conversion, dissolution
of nonclay 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.
