Impact of lithofacies categories on inflection-point behaviors in micro-nano pore-structure evolution: Implications for differential reservoir-forming mechanisms and “sweet-spot intervals” identification for marine gas shales

Abstract

Pore structure evolution characteristics and the controlling factors of organic-rich shales have received considerable attention, with recent investigations have focused on pore-type heterogeneity and pore-structure variations during the thermal maturation. A still controversially discussed issue concerns the potential inflection points of the microscopic reservoir parameters occurring at specific thermal evolution stages, and a deeper understanding and reasonable interpretation of the differential pore-structure evolution trajectory for marine shale-gas reservoirs remains a challenging task. In this study, we focus on black shale reservoirs in the Wufeng-Longmaxi Formations (Upper Ordovician-Lower Silurian) from Southwest China. An integrated analytical approach is employed utilizing lithofacies classification, FE-SEM observation and digital image extraction, fluid injection techniques, calculation of innovatively proposed predominant direction coefficients of migrated organic matter (CMOM), as well as FIB-SEM microscopy and three-dimensional (3D) reconstruction. We offer new perspectives on and constraints particularly regarding the potential effect of lithofacies on differential inflection-point behaviors in the evolution of multi-scale and organic-matter pore structures during thermal maturation. Additionally, we offer a unique perspective on the differential reservoir-forming mechanisms of the heterogeneous shale-gas reservoirs. The significant time-lag effect observed at the identified inflection points trending toward the high-quality siliceous shale reservoirs is confirmed, suggesting the strongest storage capabilities of migrated organic matter, and a most sustainable and advantageous window period for organic matter-hosted pore development. The optimal genetic conditions for disadvantageous argillaceous shales are also uncovered by assessing sensitive parameters to evaluate pore-throat connectivity. This highlights the importance of interconnected micropores facilitating adsorbed-gas micro-reservoirs and seepage properties at the appropriate thermal maturity stage in heterogeneous and porous medium. Our findings may fill significant gaps in identifying “sweet-spot” categories and “sweet window” intervals, as well as assessing favorable exploration and commercial exploitation targets for marine shale-gas reservoirs.