Characterization and modeling of sweet spots in tight sandstone reservoirs of the Jurassic Ahe Formation, Yangxia outcrop, Tarim Basin: A case study using conventional outcrop data and UAV oblique photogrammetry model

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.