A Review of the Heterogeneity of Organic-Matter-Hosted Pores in Shale Reservoirs

Abstract

Organic-matter-hosted pores are fundamental pore spaces in shale reservoirs, which largely control the expulsion and storage of oil and gas. However, the strong heterogeneity of organic pores greatly increases the complexity of the pore network in shale reservoirs, which make shale reservoir evaluation challenging. The heterogeneity of organic pores in shale reservoirs has beenone of the hottest topics of recent years. In this review, the heterogeneity of organic pores in shale reservoirs and their controlling factors are systematically summarized. The formation and evolution of organic pores in shale reservoirs are directly linked to hydrocarbon generation and expulsion,and the heterogeneity of organic pores is a result of various geological and geochemical factors. The development and heterogeneity of organic pores are basically controlled by the differences in kerogen types and maceral compositions of shale deposits, which are mainly attributed to the differencesin hydrocarbon generation capacity of different maceral compositions. Thermal maturation of organic matter is responsible for the formation and evolutionary mechanisms of organic pores and their heterogeneities. With increasing maturity, the increasing trend of pore volume and porosity diminishes. The reduction in macropore volume first appears, and the collapse of macropores could lead to an increase in micropore and mesopore volumes. An important turning point for the thermal evolution of OM is 3.5% Ro. At an Ro greater than 3.5%, the chemical structure of OM is transformed from amorphous carbon to crystalline graphite, and the hydrocarbon generation capacity of the OM has been exhausted, thus, resulting in the destruction of OM-hosted pores. The TOC content and mineral compositions of shale reservoirs affect the development and preservation of organic pores, but enhanced TOC content and brittle minerals may work against the development and preservation of organic porosity. Geological factors, e.g., compaction, diagenesis, pore fluid pressure and tectonic deformation, can also affect the organic pore structure of shale reservoirs to some extent, and their differences can enhance the heterogeneity of organic pores. On the basis of the above-mentioned understandings, this review also puts forward and discusses the problems existing in the current study of organic pore and its heterogeneity of shale reservoirs, and points out further research directions.