Research into pore evolution in Cretaceous continental shales in the Songliao Basin
LI Canxing, LIU Dongdong, XIAO Lei, JIANG Zhenxue, LI Zhuo, GUO Jing
1 State Key Laboratory Petroleum Resources and Prospecting, China University of Petroleum-Beijing, Beijing, 102249, China 2 Unconventional Petroleum Research Institute, China University of Petroleum-Beijing, Beijing 102249, China
The pore structure of shale determines the storage capacity of shale reservoirs. Therefore, studying the pore evolution process of shale is of great significance in revealing the mechanism of shale gas enrichment. Previous systematic research work
has been carried out on the pore evolution process of marine shale, but relatively little research has been done on the pore evolu-tion process of continental shale. This paper takes the low-maturity (RO=0.65%) continental shale of the Qingshankou Formation in the Songliao Basin as the research object. Through thermal simulation experiments, combined with gas adsorption, mercury intrusion capillary pressure (MICP), X-ray diffraction (XRD) and other analysis, the evolution process of the pore structure of the continental shale of the Qingshankou Formation in the Songliao Basin has been investigated. Systematic analysis, has established the pore evolution model of this continental shale. The results show that as the degree of evolution increases, the pore volume and specific surface area of shale undergoes a process of first increasing, then decreasing, and then increasing. The evolution of pores is mainly controlled by organic hydrocarbon generation and clay mineral conversion. When 0.65%＜RO≤0.86%, kerogen slowly generates hydrocarbons and combined with organic acid dissolution, some organic pores are formed, shale pore volume and specific surface area increase slightly. When 0.86%＜RO≤1.18%, montmorillonite dehydration and new pore formation will occur. This results in an increase in pore volume and specific surface area, but kerogen begins to generate a large amount of oil at this stage, and the generated liquid hydrocarbons and asphalt will fill part of the original pores, making the shale pore volume and specific surface area decrease. When 1.18%＜RO≤1.86%, the kerogen at the maturity-high maturity stage continues to generate oil, but the rate of oil generation begins to decrease. At the same time, the kerogen begins to pyrolyze and form some gas bubbles. And the bubbles further increase with the degree of evolution. During this period, the montmorillonite in the I/S (illite/smectite mixed layer) transforms to illite. The three effects promote the increase of the pore volume and specific surface area of micropores, mesopores and macropores. When RO＞2.15%, kerogen oil production stops, the liquid hydrocarbons generated begin to crack and generate gas, forming a large number of bubble pores, and the pores further increase in the later period, The phenomenon of pore fusion occurs, and at the same time, the conversion of montmorillonite to illite in I/S leads to a substantial increase in pore volume and pore specific surface area.
