Digital Outcrop Modeling and Structural Characterization of Continental Shale Reservoirs: A Case Study of the Gulong Shale, Songliao Basin, China

Vollständiger Abstract

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Continental shale reservoirs exhibit pronounced multi-scale heterogeneity, with reservoir quality governed by the interplay of lamina assemblages, bedding continuity, and lithological spatial variability. Conventional digital outcrop modeling (DOM) primarily targets geometric reconstruction and three-dimensional (3D) visualization of sedimentary bodies, which is insufficient for the fine-scale structural characterization and quantitative modeling required for shale reservoirs. Here we present a Digital Shale Outcrop Modeling method (DSOM) tailored to continental shale reservoirs, exemplified by the Gulong Shale in the Qingshankou Formation (Upper Cretaceous) of the Songliao Basin, northeastern China. DSOM integrates six sequential modules: digital outcrop reconstruction, digital section interpretation, virtual well construction, virtual well correlation, 3D structural modeling, and parameter extraction. A high-precision digital outcrop model covering 0.369 km2 was constructed from 1885 calibrated UAV images (DJI Mavic 3 Enterprise) using Structure-from-Motion (SfM) photogrammetry, yielding derived products including a digital outcrop model (DOM), digital surface model (DSM), digital elevation model (DEM), orthomosaic, and dense point cloud. Seven virtual wells were extracted along the outcrop strike, and a unified lithological classification comprising seven lithotypes was established. A regionally persistent rusty-yellow ferruginous siltstone layer served as a marker bed for virtual well correlation. Results reveal a distinct vertical lithological transition: the section above the marker bed is dominated by muddy deposits, with black mudstone and dark-gray silty mudstone collectively accounting for 52.84% of the total area, whereas the section below the marker bed exhibits a marked increase in silt-grade material, with gray siltstone reaching 32.78%. This vertical evolution reflects a depositional shift from relatively high-energy to low-energy conditions. Using the virtual wells as conditioning data, 3D lithological probability models for all seven lithotypes were constructed via Sequential Indicator Simulation (SIS), achieving quantitative representation of lithological spatial distribution and lateral variability under bedding constraints. Our results demonstrate that DSOM effectively converts outcrop digital information into reservoir structural data, providing reliable constraints for multi-scale structural characterization, 3D geological modeling, and heterogeneity evaluation of the Gulong Shale. More broadly, DSOM establishes a methodological framework for digital characterization of fine-grained sedimentary reservoirs, bridging the gap between outcrop-scale observations and subsurface reservoir modeling.