Multiscale pore network characterization and genetic mechanisms of fluid mobility heterogeneity in deepwater globigerinid limestones

Deep-water globigerinid limestone gas reservoirs differ from conventional carbonates due to their complex fabric and strong multiscale heterogeneity. This study systematically discusses the intrinsic control of primary sedimentary fabrics on macroscopic fluid mobility and pore topological connectivity. An integrated multiscale characterization framework was established, using X-ray diffraction (XRD), cast thin sections, scanning electron microscopy (SEM), mercury injection capillary pressure (MICP), nuclear magnetic resonance (NMR), and micro-computed tomography (Micro-CT) data. The results indicate that grain-supported lithofacies exhibit a rigid framework, maintaining high permeability (8.23 mD) through a unique micropore bridging effect with a high fractal dimension (Dmicro ≈ 1.37). Conversely, matrix-supported facies display bioclasts floating in a clay matrix, causing network disconnection (0.69 mD) and high irreducible water saturation (76%). Coupled sedimentary-diagenetic analysis reveals that stress shielding effectively protects critical micro-throats from compactional closure in rigid fabrics, whereas the plastic rheology of clay filling creates severe flow barriers. This confirms that topological connectivity, rather than porosity magnitude, is the key factor governing productivity, and identifying the “micropore bridging” mechanism as a vital foundation for reservoir quality in deep-water mixed systems. These findings provide theoretical support for sweet spot prediction and differentiated acid fracturing strategies.