Evolution characteristics and model of nanosclae pores in organic-rich shale during thermal maturation

         Shale is an unconventional and complex oil-bearing system with the trinity of source, reservoir and cap. The coupling evolution of thermal maturation hydrocarbon generation, diagenesis and nanoscale porosity is the key scientific problem affecting the accumulation and accumulation of shale gas. In this research, the low matured marine shale of Mesoproterozoic Xiamaling shale in Zhangjiakou, Hebei were selected to conduct the thermal simulation experiments, then the pyrolysis products at each temperature point were recovered and were subject to an ongoing multidisciplinary analytical program. The simulation experiment results show that, in the process of simulated temperature increasing, the maturity of the shale sample is risen generally. the role of hydrocarbon expulsion of the shale at the same time, also to form the inner groove and the shrinkage hole edge groove organic matter more, side by side out of a large number of organic acid, the acid fluid for inorganic pore formation and evolution of simulated sample plays an important role in promoting, It also affects the diagenetic evolution of mud shale. Along with the hydrocarbon generation and expulsion, shale also forms a large number of internal multi-pore and contractive margin pores, and expel a large number of organic acids. These acidic fluids play an important role in promoting the generation and evolution of inorganic pores in the simulated samples, and also affect the diagenetic evolution process of shale.

        The increased temperature accelerates the dissolution of unstable brittle minerals and produces dissolution pores, promotes the transformation of clay minerals, and accelerates the formation and development of clay mineral pores. The nanoscale pore diameter did not change significantly during the simulation process, while the pore volume decreased first and then increased, reaching the minimum and maximum values at 350°C and 650°C, respectively. The surface area of micropores and mesoporous pores firstly decreased and then increased, reaching the minimum value at 350°C, while the surface area of macropores firstly increased and then decreased, reaching the minimum value and maximum value at 350°C and 650°C, respectively(Figure 1).

Figure 1. The pore volume and surface area variation characteristics of micropore (a，a'), mesopore(b, b'), macropore(c, c') during the increase of the thermal temperature.

         The diagenetic evolution during simulated temperature rise can be divided into four stages, and the main diagenetic types are dissolution, clay mineral transformation, thermal maturation hydrocarbon generation, compaction and recrystallization. In our research, the diagenetic evolution process and pore evolution model of shale were roughly divided, and the coupling evolution model of thermal mature hydrocarbon generation, diagenesis and pore structure of shale was established based on thermal simulation experiment (Figure 2).

Figure 2. Comprehensive diagram of the diagenetic evolution sequence and pore evolution model based on the hydrous pyrolysis experiment

 

         The coupling evolution model  provides qualitative and quantitative characterization and evaluation methods for hydrocarbon generation, diagenesis and nanoscale pore structure evolution of organic-rich shale.