Abstract: In a case study in the lower part of the first member of the Liushagang Formation (Elx1) in the middle of the steep slope zone in the Weixinan sag, we examined the sedimentary characteristics, controlling factors and distribution features of deep-water fan formed by hyperpycnal flow through analyzes of 3D seismic information, well logging and core drilling data and laboratory measurements. The results show that hyperpycnites are distributed from the steep slope to the bottom of lacustrine basin, where hyperpycnal flow developed sequentially straight and meandering channels and lobes which expand 15 km long and 50100 m wide. The seismic reflections of hyperpycnites are obviously stronger than that of the surrounding deep-water mudstones. Hyperpycnites are mainly composed of fine-grained sandstone, siltstone and small amounts of pebbly sandstone; terrigenous biolastics (plant material) are also common. The grain size accumulation curve is mainly overarching and the sample point distribution is roughly parallel to the baseline of C=M, showing the characteristics of gravity flow deposits. A bed of sandstone is usually composed of paired inverse and normal grading sequences, with the coarsest grain depositing in the middle of the bed. Rip-up mud clasts are typical with imbrication. The intrasequence weak erosional contacts are also obvious. In addition, hyperpycnal flow developed sedimentary structures of massive and parallel bedding, Hummocky cross-stratification and climbing sand ripples by bed- and suspended-load transportation. Since sedimentary characteristics and distribution of hyperpycnites are determined by source-to-sink system, magmatic rocks originated from eastern Wanshan uplift provided the material basis of hyperpycnites, while strong tectonic movements, steep slope, wet-half humid climate and relatively low density of water are the main controlling factors for the development of hyperpycnites. Our discovery of hyperpycnal flow deposits in the study area not only enriches our knowledge of sedimentary environment in the steep slope belt in lacustrine basin, but also provides theoretical guidance for further hydrocarbon exploration in steep slope regions.

Key words: hyperpycnites, sedimentary characteristics, controlling factors, steep slope, lower part of the first member of the Liushagang Formation, Weixinan sag