Investigation of formation and evolution of organic matter pores in marine shale by helium ion microscope: An example from the Lower Silurian Longmaxi Shale, South China

Abstract

Using a helium ion microscope (HIM), we carefully observed and analyzed organic matter (OM) pores in several Longmaxi shale samples collected from the Upper Yangtze Platform, South China. The results indicate that the formation process of OM pore essentially involves gradual expansion and connection of defects/space within OM. This process, which is divided into the four stages of formation, expansion, connection, and consolidation, can involve pore scales ranging from a few nanometers to over a micrometer under HIM. More specifically, spotted spherical pores first form within OM, then expand until they connect to adjacent pores. Within large areas of such connectivity, “pit” structures eventually form, preliminarily defined as OM pores that are significantly larger than surrounding pores, essentially being greater than 100 nm in size. As the final stage of OM pore evolution, the shape and size of “pit” structures can be limited by the framework of mineral particles around or within the OM. Additionally, we found that surrounding pores are smaller when “pit” structures develop, perhaps because removal of side chains of OM tends to occur on the inner surface of the formed pores, leading them to rapidly expand, or because OM particles have differing compositions or structures, some of which may be unfavorable to pore development.