Organic Pore Structure Evolution Characteristics Of Continental Shale In Ziliujing Formation In Northeastern Sichuan Basin

Pore structure significantly affects the occurrence state of shale gas as well as the storage and migration of shale gas within shale reservoirs.Organic matter(OM)pores can provide abundant storage space for shale gas.However,there are different types of OMs with different compositions and structures in continental shale and the pore structure evolution characteristics lack direct observations.In this study,thermal simulation experiments are conducted on continental Jurassic Ziliujing Formation shale samples with a low thermal maturity selected from the outcrop in northeastern Sichuan Basin.Field emission–scanning electron microscopy(FE-SEM)is used to reveal the consecutive pore structure changes of a specific OM at different thermal maturities.The FE-SEM images processed by image processing software are combined with N₂adsorption and high pressure mercury intrusion porosimetry(HPMIP)to clarify the pore structure evolution characteristics.Our results show that OMs developed in Ziliujing Formation shale can be divided into four types based on their morphological characteristics and C/O ratios.The pore structure evolution process is closely related to the processes of hydrocarbon generation,migration and thermal cracking.More specifically,the filling of pores by generated hydrocarbon observed at lower temperature caused the decrease of macropore volume in shale and these filled macropores were released at higher temperature by thermal cracking of hydrocarbons.In addition,there were no OM pores larger than 10nm newly produced during thermal simulation experiments,which indicates that the development of OM pores larger than10nm is largely dependent on the inherent composition and structure of their parent OM.The outcomes of this study have significant theoretical and practical values for clarifying the occurrence state of continental shale gas and improving the shale reservoir evaluation.