Diagenetic evolution sequence and pore evolution model of Mesoproterozoic Xiamaling organic-rich shale in Zhangjiakou, Hebei, based on pyrolysis simulation experiments

Abstract

Organic-rich shale is an unconventional and complex petroliferous system that integrates a “source-reservoir-cap”, and the coupled evolution of hydrocarbon generation, diagenesis and nanoscale pores is the key problem that affects shale gas accumulation. Current research methods for this issue mainly include direct observation and physical simulation. Direct observation ignores the heterogeneity and regional differences of the natural samples, and physical simulation mostly lacks an intuitive characterization and cannot clearly show the relationship between minerals and pore evolution characteristics in the same region. To avoid the effect of heterogeneity on the researched samples and clearly and intuitively reveal the coupled evolutionary relationship among the hydrocarbon generation of thermal maturation, diagenesis and nanopore structures in shale, this research included hydrous pyrolysis experiments on low mature marine shale samples to achieve various maturities and diagenesis stages. The pyrolysis products at each stage were recovered and subjected to an ongoing multidisciplinary analytical program. The results show that an increased temperature intensifies the evolution of dissolution pores in unstable brittle minerals, promotes clay mineral conversion, and accelerates the development of clay mineral pores and organic pores. The pore volume (PV) of the nanoscale pores reached its minimum at 300 °C, increased to its maximum at 500 °C and decreased thereafter. The surface area (SA) of the nanoscale pores reached their minimum at 300 °C and then continuously increased, while those of macropores reached their maximum at 500 °C and then decreased. The diagenesis of the pyrolysis products were mainly dissolution, clay mineral transformation, the thermal maturation of organic matter (OM), compaction and cement filling. The diagenetic evolution was divided into four stages, and the diagenetic evolution sequence and pore evolution model of the shale were established. Hydrocarbon generation, diagenesis and nanoscale pore evolution have synergistic effects, which have great significance for shale reservoir evaluation and shale gas accumulation, exploration and exploitation. The conceptual evolution model provides a quantitative prediction method for hydrocarbon generation, diagenesis and nanoscale pore variations.