3D Geological Modeling of Gravity Flow Fans Based on Field Outcrop Measurements and Ground Penetrating Radar

Abstract

Deep-water gravity-flow depositional systems are characterized by intricate internal architectural configurations and pronounced heterogeneity. Traditional outcrop studies rely heavily on qualitative observations and often fail to capture the sub-seismic 3D spatial heterogeneity required for precise hydrocarbon exploration. This study leverages an integrated multi-geophysical framework to achieve a high-resolution, quantitative characterization of internal structures within gravity-flow fan units in field outcrops. Focusing on the Upper Ordovician Lashizhong Formation at the Beishan outcrop in Wuhai, northwestern Ordos Basin, the research integrates Terrestrial Laser Scanning (LiDAR) to construct a 3D topographic framework and uses Ground-Penetrating Radar (GPR) to probe subsurface architecture. Furthermore, Full-Waveform Inversion (FWI) is employed for the high-resolution reconstruction of relative permittivity, culminating in the generation of a 3D geological model via Sequential Gaussian Simulation (SGS). Six primary lithofacies were identified: (1) massive-to-graded medium-to-fine-grained sandstone, (2) graded fine- to silty sandstone, (3) parallel-laminated fine sandstone, (4) climbing ripple-laminated siltstone, (5) wave ripple-laminated siltstone, and (6) horizontally bedded mudstone. Geometrically, six channel–levee architectural stages and one sheet-lobe stage within a mid-fan setting. Quantitative analysis shows that channel axes maintain a high net-to-gross (N/G) ratio of 0.82 with 92% connectivity, whereas levee and lobe margins exhibit a significantly reduced N/G of 0.28 and 42% connectivity. The overall cross-validation accuracy of the 3D model reached 86.4%. This research provides an objective, quantitative technical framework for 3D spatial characterization and reservoir modeling of complex gravity-flow systems.