Depositional characteristics of fluvial facies in gentle slope zone of lacustrine rift basins: The third member of the Dongying Formation in the Wen’an Slope of Baxian Sag, Bohai Bay Basin, China

Abstract

We have integrated core, well logs, and seismic data to analyze fluvial types and reconstruct sediment dispersal patterns of the Dongying Formation in the Wen’an Slope, Bohai Bay Basin (China). We analyzed four meandering-braided fluvial successions from the Dongying Formation in the Wen’an Slope. The results indicate that the fluvial systems are dominantly northeast–southwest and consist of channel, channel bar, and flood plain subfacies, which can be further subdivided into six microfacies including braided channel, meandering channel, sandy channel bar, muddy channel bar, crevasse fan, and floodplain. With a paleoflow direction, [Formula: see text] ranges from 40° to 65°, the river belt width [Formula: see text] is estimated to be 1.47–2.64 km, whereas the main channel width varies from 0.02 to 0.29 km and its thickness/bankfull depth [Formula: see text] ranges from 1.2 to 6.4 m. The differential subsidence of rift basin during fault-depression transition period is the main control on fluvial evolution and sand body distribution. The climatic effect causes changes in river flow. During the flood season, river flow and flow rate are large, and early sediments and sand bodies are reconstructed. Lateral erosion and undercutting are very strong in this period. In the dry season, the river is dominated by weak lateral erosion. Our study also suggests that the development of thick mudstone in the upslope may limit hydrocarbon charging and migration and thick channel and channel bar deposits in the middle and downslope cross cut by faults, which are conduits for migration are likely to be the most potential reservoirs in the Bohai Bay Basin.