Pore characteristics and influencing factors of different types of shales

Shales in China mainly fall into three categories, namely marine, transitional and lacustrine shales. Various shales have different pore characteristics. We studied the formation, occurrence, pore types and morphological characteristics of micropores, mesopores and macropores in different types of shales through the approaches of gas adsorption and mercury injection. Our study found that the marine shale samples have larger specific surface area and pore volume, providing a good hosting space for adsorption gas and free gas. Furthermore, marine samples are dominated by ink-bottle pores, which are conducive to adsorption of shale gas. Continental and transitional pore types are mainly open pores, characterized by good pores' connectivity that is conducive to the migration of shale gas. The main factors controlling the pore characteristics of the three types of shales were analyzed by investigating specific surface area, pore volume, organic content, clay mineral content, maturity and burial depth. Our study demonstrated that specific surface area and pore volume development have a good correlation with organic matter in the three types of shales. Types of organic matter can affect the development of organic pores. The discrepancy in occurrence of organic matter leads to different correlations between the three types of shales and clay minerals. On the other hand, maturity impacts on pores of shales in two aspects: organic matter and clay mineral transformation. Meanwhile, compaction and dissolution of organic acid in hydrocarbon-generation process are proved to be especially important to pore structure transformation.