Controls on Pore Connectivity and Evolution in High Total Organic Carbon Permian Wujiaping Shales, Eastern Sichuan Basin

Pore connectivity is an important factor controlling fluid flow behavior and the ultimate recovery in shale reservoirs. However, the controlling factor and dynamic evolution of pore connectivity across thermal maturity stages remain poorly understood for gas-producing shale reservoirs. In this work, high-organic-matter shale from the second member of Wujiaping Formation (Wu-2), the Eastern Sichuan Basin, is systematically examined to characterize its pore connectivity, discuss controlling factors, and establish evolutionary models. Complementary methods, including X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), gas physisorption (N2/CO2), mercury intrusion capillary pressure porosimetry (MICP), spontaneous imbibition (SI), and nano-CT imaging, were employed. Results show that four lithofacies, including highly organic-rich siliceous shale (HOS), highly organic-rich mixed shale (HOM), organic-rich mixed shale (ORM), and moderate- to low-organic-rich calcareous shale (MLC), are identified in the Wu-2 shale reservoir. The HOS shales demonstrate superior reservoir quality and are characterized by the most developed multiscale pore networks, highest connected porosity, and good pore network connectivity, whereas MLC shales exhibit the lowest connected porosity and poor pore connectivity. In contrast, the connected porosity and pore connectivity of HOM and ORM shales are intermediate between those of the two shales. High total organic carbon (TOC) content and sapropelic-rich kerogen are two primary drivers in generating an extensive secondary organic pore network, while a rigid microcrystalline quartz aggregate framework is crucial for its preservation. Conversely, carbonate cementation and humic-rich kerogen significantly impair pore connectivity. The pore connectivity of Wu-2 shale first increases and then decreases as thermal maturity levels increase. During the high-maturity stage (Ro = 1.0–2.0%) due to hydrocarbon generation and expulsion as well as the rapid development of organic pores, the pore connectivity of shale reaches its optimum. However, in the overmaturity stage (Ro > 2.0%), because of the depletion of hydrocarbon generation from organic matter, the cessation of the increase in organic pores, and intense compaction from overlying strata, pore connectivity of shale significantly deteriorates. Finally, a pore connectivity evolutionary model for different shale lithofacies is established, which reveals that, compared to other shales, HOS shales develop and maintain a highly effective pore network at different stages of thermal evolution. These findings serve as an important reference for reservoir evaluation and sweet spot prediction in the Permian Wujiaping shale reservoir.