Reservoir Space Characteristics, Pore-Throat Structure, and Microscopic Hydrocarbon Occurrence of Chang 73 Submember Tuffs: A Two-Well Case Study from the Ordos Basin, China

Tuff interlayers within organic-rich shale contain diverse pore and fracture systems that influence microscopic hydrocarbon occurrence. Tuff samples from wells Y61 and Y245 in the Chang 73 Submember of the Yanchang Formation, Longdong area, Ordos Basin, China, were investigated using thin-section petrography, field-emission scanning electron microscopy, high-pressure mercury intrusion (HPMI), nuclear magnetic resonance (NMR), micro-computed tomography (micro-CT), fluorescence microscopy, and confocal laser scanning microscopy (CLSM). The samples contain abundant vitric and crystal fragments within a fine-grained tuffaceous matrix. Reservoir spaces include dissolution, interparticle, intraparticle, intercrystalline, and locally developed organic matter-hosted pores, together with fractures and microfractures. Dissolution features indicate local secondary-pore development, whereas close particle packing and mineral filling are associated with reduced open space. HPMI-derived equivalent entry-throat radii at 50% reference mercury saturation are 0.012 and 0.016 μm for Y61-2100.61 and Y245-2116.24, respectively. In Y245-2116.24, NMR signal reductions relative to the initial saturated state are 6.31% and 8.87% after the first and second successive centrifugation stages, respectively. Micro-CT reveals heterogeneous pore clusters and locally discontinuous bands, with an image porosity of 4.47%. Fluorescence is associated with pores and fractures, while 3D CLSM reveals variations in fluorescence-channel volume fractions among three selected microdomains. Reservoir-space heterogeneity in the examined tuffs is associated with variations in volcaniclastic composition, dissolution, mineral filling, and fracture preservation.