Impact of laminae characteristics on pore-fracture connectivity in the Wufeng-Longmaxi shale

The pore-fracture connectivity in unconventional shale systems is predominantly governed by laminae, necessitating comprehensive investigations through various perspectives. Thus, to quantitatively assess the influence of laminae and their characteristics on pore-fracture connectivity, optical microscopy and field emission scanning electron microscopy combined with Wood's metal impregnation are employed, integrated with mercury intrusion capillary pressure analysis, gas permeability testing, and nanoindentation experiments on Wufeng-Longmaxi marine shale samples from the southern Sichuan Basin, China. The results indicate that the level of pore-fracture connectivity (from good to poor) in three types of recognized lamina from petrographic observations of the samples follows: silty-organic rich laminae (η = 0.42–0.47) > silty-clay rich laminae (η = 0.36) > clay rich laminae (η = 0.20). Silty-organic rich laminae exhibit the best connectivity due to the higher abundance of organic matter, well-developed organic pores, and interlaminar fractures that facilitate their connectivity. In contrast, clay rich laminae demonstrate the poorest connectivity due to their low organic matter content and the predominance of isolated, elongated pores in clay minerals. Furthermore, permeability anisotropy of shale is primarily influenced by mineral composition and the degree of microfracture development where, silty-organic rich laminae exhibit the most extensive distribution of microfractures, supported by a rigid framework composed of brittle minerals. This results in the lowest pore-fracture compressibility coefficient, which helps maintain superior connectivity and fluid flow pathways. Nanoindentation experiments demonstrated that silty-organic rich laminae will have the highest elastic modulus (E > 30 GPa) and hardness (H > 2 GPa). With increasing clay content, these mechanical properties exhibit progressive reduction, promoting enhanced susceptibility to pore collapse and fracture closure, thereby significantly impairing pore-microfracture connectivity. Overall, the pore-fracture connectivity of different types of laminae controls the gas supply from matrix to the fractures, directly affecting the production and recovery rate of shale gas. During exploration, the silty-organic rich laminae in the middle-lower part of the TST of the Longmaxi Member 1 should be prioritized as the sweet spot. These findings have significant implications for evaluating favorable spots in shale reservoirs, for an optimized field development plans, specifically hydraulic fracturing, for higher gains and resource utilization.