New Insights into Controlling Factors of Pore Evolution in Organic-rich Shale

Abstract

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.

Fine characterization of pore evolution of oil shale during thermal simulation

“…Guo, Jia, et al, 2017;Q. Guo, Wu, & Ren, 2017;Xu et al, 2021), they agreed that the thermal evolution of OM played an important role in the evolution of pores in shale (Chen & Xiao, 2014;Yu et al, 2020). Pore evolution had a great influence on the physical properties of shale reservoirs (H. Guo, Jia, et al, 2017;Q.…”

“…However, the generation and migration of pyrolysis products caused the connectivity and porosity of pores increasing (Q. Wang et al, 2010). Although scholars have not reached a consensus on the regularity of pore evolution (Curtis et al, 2012; H. Guo, Jia, et al, 2017; Q. Guo, Wu, & Ren, 2017; Xu et al, 2021), they agreed that the thermal evolution of OM played an important role in the evolution of pores in shale (Chen & Xiao, 2014; L. Sun et al, 2015; Yu et al, 2020). Pore evolution had a great influence on the physical properties of shale reservoirs (H. Guo, Jia, et al, 2017; Q. Guo, Wu, & Ren, 2017; Ko et al, 2016).…”

Review of the Effect of Diagenetic Evolution of Shale Reservoir on the Pore Structure and Adsorption Capacity of Clay Minerals

“…182−184 Dissolution in shale gas reservoirs mainly occurs in carbonate minerals or potassium feldspar, but the organic acid formed by organic matter hydrocarbon generation also has a certain dissolution effect on clay minerals. 5,185,186 Metasomatism refers to the dissolution of original minerals and the formation of new minerals, which is generally considered to have little impact on the pore structure of clay minerals. As mentioned above, the role of clay content in CH 4 adsorption capacity decreases during shale diagenesis.…”

“…Cementation, dissolution, metasomatism, and recrystallization play a role in pore structure transformation. Recrystallization includes the transformation of amorphous siliceous minerals to authigenic quartz and the secondary enlargement of clastic quartz crystals, both of them will reduce the pore space of inorganic minerals. ,, Dissolution is constructive to the formation of pore space, including the dissolution pores in clay minerals and the dissolution of carbonate and siliceous cements in clay mineral pores. − Dissolution in shale gas reservoirs mainly occurs in carbonate minerals or potassium feldspar, but the organic acid formed by organic matter hydrocarbon generation also has a certain dissolution effect on clay minerals. ,, Metasomatism refers to the dissolution of original minerals and the formation of new minerals, which is generally considered to have little impact on the pore structure of clay minerals.…”

Pore-Scale Study of the Wetting Behavior in Shale, Isolated Kerogen, and Pure Clay

“…However, this wettability evaluation is debatable and under certain circumstances even erroneous in the application of shale . Shale bears a large lithological heterogeneity under the joint impacts of clay minerals and OM components, − leading to a large variation in wettability or wetting behavior performance. OM pores typically present a hydrophobic performance. ,, Kaolinitic mudrocks show a hydrophobic preference, whereas shales rich in illite and smectite present a more hydrophilic preference .…”