Given limited studies on the origin and spatial distribution patterns of matrix-type reservoir sweet spots in tight sandstones, we innovatively develop a methodology that combines conventional outcrop surveys with modern unmanned aerial vehicle (UAV)-based oblique photogrammetry. Specifically, hierarchical constraint-based architectural characterization approach is integrated with three-dimensional (3D) geological modeling and a range of modern analytical and testing techniques, including conventional porosity and permeability measurements, thin section observations, scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), elemental analysis based on X-ray fluorescence (XRF)/X-ray diffraction (XRD), and computed tomography (CT) scanning. Using these methods, we systematically perform lithofacies identification, interpret representative outcrop sections, and analyze the differential patterns of reservoir quality. Furthermore, we conduct 3D geological-engineering modeling and examine the spatial distribution pattern of sweet spots. The research results indicate that the integrated application of traditional outcrop characterization and UAV-based oblique photogrammetry significantly enhances both the dimension and precision of sedimentological investigation. A total of 12 lithofacies types are identified within the braided river delta in the outcrop area. Among these, distributary channels and distributary sandbars constitute the primary sand body frameworks, while mudstones deposited in interdistributary bays and abandoned channels act as key baffles and barriers. Three patterns for the quality difference of tight sand reservoirs are identified: (1) a filled, positive-rhythm pattern, typically characterized by upward-decreasing porosity within individual braided channels; (2) a lateral-aggradation-controlled migrating and stacking pattern, which exhibits a distinct progressive decrease in porosity along the lateral aggradation direction of channels and, accordingly, forms a complex, multi-stage, superimposed positive-rhythm structure; (3) a composite pattern that integrates the characteristics of the preceding two patterns. What’s more, 3D geological and engineering parameter models are constructed based on the reservoir architecture bounding surface algorithm. Using high-precision digital outcrop data acquired by UAV-based oblique photogrammetry, we successfully develop the 3D models of key lithofacies-controlled parameters, including porosity, Poisson’s ratio, Young’s modulus, and the brittleness index. These models comprehensively reveal the heterogeneity of tight sandstone outcrops at multiple scales. Based on the evaluation criteria covering geological and engineering parameters, the spatial distribution patterns of reservoir sweet spots in tight sandstones are identified, and the differential spatial distribution characteristics of various sweet spot types are further examined. The results of this study provide an important theoretical basis and practical guidance for future exploration and exploitation of tight-sand hydrocarbon resources.

Fine-grained sedimentary rocks, in which low-resistivity oil and gas reservoirs are well developed, have become a significant target in unconventional hydrocarbon exploration. The study serves to reveal the genetic mechanism of typical lowresistivity oil reservoirs of fine-grained sedimentary rocks in applying the data from logging, seismic and thin-section electron microscopy (EM), as well as well facies-controlled reservoir parameter distribution, while proposing three sweet-spot distribution patterns under low resistivity. The research results are shown as follows. First, the origin of low-resistivity reservoirs mainly includes the large specific surface area, high content of mixed layer illites/smectites, well-developed micropore network and high bound water content, high-salinity water and high content of conductive minerals. Second, we propose three typical types of fine-grained sedimentary rocks with low resistivity, including tight low-resistivity oil reservoir with high mud content (i. e., tight reservoir of low resistivity), low-resistivity oil reservoir with high content of conductive minerals (i. e., conducive mineral-dominant reservoir of low resistivity) and water network low-resistivity oil reservoir with well-connected loose sandstones (i. e., well-connected water network conductive reservoir of low resistivity). Meanwhile, their genetic mechanisms are analyzed built on this. Third, the sweet-spot distribution patterns in subtle low-resistivity reservoirs are clarified. The tight reservoir of low resistivity has the sweet spots mainly developed in those of relatively higher resistivity; the conductive mineral-dominant type has the sweet spots mainly grown in the dissolved section of the dolomitic siltstone matrix and the section with well-developed dolomitic mud/shale fractures; meanwhile, the well-connected water network conductive reservoir of low resistivity has the sweet spots mainly developed in the ultra-fine-grained rocks with dual-mode pore structure. The results are of great significance to guiding the exploration of fine-grained sedimentary oil/gas reservoirs.