Factors influencing the evolution of shale pores in enclosed and semi-enclosed thermal simulation experiments, Permian Lucaogou Formation, Santanghu Basin, China

Abstract

The microscopic pore structure of organic-rich shales is an important factor affecting the enrichment of shale gas; however, the characteristics of pore structure changes during thermal evolution are unclear, and the factors influencing the pore evolution remain controversial. High-temperature and high-pressure semi-enclosed and enclosed pyrolysis experiments were conducted in the low-mature oil shale of the Middle Permian Lucaogou Formation in the Santanghu Basin, Xinjiang, to reveal the evolution characteristics of shale pores from the low-mature to high- and over-mature stages. The results show that in the low-mature to mature stages, retained oil/residual bitumen has a clogging effect on shale pores in both systems, and the extra pressure destroys the structure of the mesopores and macropores. However, in mature to high- and over-mature stages, the pore pressure generated by the cracking of oil and gas transforms some micropores to mesopores and macropores in both systems. In semi-closed systems, large amounts of pre-oil and hydrocarbons are discharged periodically, which is conducive to the development of kerogen pores; however, the pore contents of solid bitumen and pyrobitumen are relatively small. In other cases, organic pores in solid bitumen and pyrobitumen in enclosed systems are more developed.