Quantitative reconstruction and sedimentary response of a Jurassic paleodrainage system

Fluvial facies are excellent reservoirs for fluids, critical for both carbon sequestration and hydrocarbon exploration. This study focuses on the Lower Jurassic fluvial deposits on the eastern flank of the Chepaizi Uplift in the Junggar Basin, addressing two key questions: (1) How to reconstruct the morphology of ancient river channels? (2) How to calculate the parameters of these channels (widths, lengths, and distribution ranges)? To achieve these objectives, we developed a quantitative paleodrainage reconstruction model guided by seismic sedimentology and GIS analyses, integrating 3D seismic data and a modern fluvial analog from Daqing Mountain region in Inner Mongolia. Using this method, we reconstructed the Lower Jurassic paleodrainage system and estimated key paleochannel parameters. Key results include: (1) V-shaped valleys are associated with fluvial erosion and high-energy depositional environments, whereas U-shaped and W-shaped valleys develop in low-energy settings. (2) The Lower Jurassic strata are divided into source, transport, and sink units, further subdivided into four sub-source-to-sink zones (P1-P4). (3) Quantitative analysis reveals that the widths of the paleochannels in the study area range from 330 to 1520 m, with lengths ranging from 2 to 9 km. The southern system features wider paleochannels and a greater sediment transport capacity. The areas of the piedmont alluvial fans in P1-P4 are 25.63 km², 37.69 km², 17.79 km², and 41.43 km², respectively. U-shaped or W-shaped channels are associated with larger depositional areas in these alluvial fans. This study provides practical guidance for hydrocarbon exploration in unexplored Jurassic areas of the Chepaizi Uplift. Meanwhile, the methodology can be applied to reconstructing paleodrainage systems in other terrestrial basins.