Formation and Evolution of Complex Pore-Fracture Systems in Shale Gas Reservoirs: Insights into Controlling Mechanisms

Abstract

Lacustrine fine-grained sedimentary rocks exhibit various lithofacies types and strong multiscale heterogeneity, encouraging us to investigate multistage characteristics of the panoramic pore-microfracture evolution process and potential triggering mechanisms of continental shale reservoirs. We present new results here from organic geochemistry analysis, X-ray diffraction, total organic carbon (TOC) analysis, the R₀ test, rock pyrolysis analysis, field emission scanning electron microscopy, low-pressure CO₂ and N₂ adsorption, high-pressure mercury injection (MIP), nuclear magnetic resonance (NMR), and spontaneous imbibition tests. First, we conducted thermal simulation experiments using low-mature shale samples with an R₀ value of 0.67%, aiming to innovatively unravel the entire dynamic evolution process of the micropore-fracture system in continental shales. Multitemperature thermal simulation investigations on naturally low-mature shale samples show that (1) the extensively developed interparticle pores and microfractures are conducive to the formation of complex and heterogeneous pore-fracture network systems. (2) The multistage evolution process of the pore-fracture system in continental shale reservoirs is triggered by differential hydrocarbon generation potential of maceral components, clay mineral transformation, and catalysis processes. (3) Four stages of the pore-fracture system evolution of the shale reservoir were identified, respectively occurring in R₀ ≤ 0.9, 0.9% < R₀ ≤ 1.6, 1.6% < R₀ ≤ 3.0%, and R₀ > 3.0%. This study laid a foundation for future research into differential diagenetic and reservoir-forming mechanisms of continental shale reservoirs, offering new insights into accurate prediction and comprehensive evaluation of the lacustrine shale gas “sweet spot”.