Contribution of lamellation fractures to porosity and permeability of shales: A case study of the Jiaoshiba area in the Sichuan Basin, China

Lamellation fractures are an important type of natural fracture in marine shales. These fractures affect the hydraulic connectivity of shale reservoir, however, their contribution to porosity and permeability is poorly understood. In this paper, we investigate the contribution of lamellation fractures to porosity and permeability of shale reservoirs in the Wufeng-Longmaxi Formation in Sichuan Basin. To do so, several analytical methods including scanning electron microscopy (SEM), CO2 and N2 adsorption experiments, high-pressure mercury injection (MICP), and permeability measurements under different confining pressures were combined. Results showed that the contribution of micro-pores, meso-pores, macro-pores, and fractures to pore volume is 29.1%, 35.7%, 12.6%, and 22.6%, respectively lamellation fractures can increase porosity 6%-10%. Besides, filling materials of the lamellation fractures, anhydrite, pyrite, and organic matter (OM), can improve shale petrophysical properties. Furthermore, it was found that lithofacies would cause different pressure sensitivities in permeability with clay rich shale being the highest and silty shale the lowest. In addition, samples with lamellation fractures exhibited higher pressure sensitivity while permeability steadily decreases with confining pressure, with its decreasing rate gradually slowing down in separate phases. Ultimately, lamellation fractures can increase horizontal permeability three at 5 MPa and two at 50 MPa of confining pressure. When compared to the actual formation pressure at 30 MPa, the permeability of shale samples with lamellation fractures is much higher than that without lamellation fractures. This study provides direct evidence that subsurface lamellation fractures can improve the storage space and permeability of shale reservoirs.