Contribution of Lamellation Fractures to Porosity and Permeability of Marine Shales: A Case Study of the Jiaoshiba Area in the Southeastern Sichuan Basin, China

Lamellation fractures are an important type of natural fracture in marine shales. These fractures affect the enrichment and production of shale gas, however, their contribution to porosity and permeability is poorly understood. In this paper contribution of lamellation fractures to porosity and permeability in shale reservoirs, taking the marine shale of the Wufeng Formation-Longmaxi Formation in the Jiaoshiba Area in the southeastern Sichuan Basin as an example has been investigated. To do so, sveral analytical methods including scanning electron microscopy (SEM), CO 2 and N 2 adsorption experiments, high-pressure mercury injection (MICP), and permeability measurements under different confining pressures were combined. Results showed that contribution of micro-pores, meso-pores, macro-pores, and fractures, to pore volume is 29.1%, 35.7%, 12.6%, and 22.6%, respectively while, 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. Furthremore, it was found that lithofacies would cause different pressure sensitivities in permeability with clay rich shale to be 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 comparing to the actual formation pressure at 30 MPa the permeability of shale samples with lamellation fractures was found around 0.497mD which is is much higher when lamellation fractures are abscent. Collectively, this study provides evidence that lamellation fractures will improve storage and flow capacity of shale gas reservoirs.