The Characteristics of Deep-Water Gravity Flow in the Sublacustrine Fan of the Upper Triassic Yanchang Formation in the Huachi Area, Ordos Basin

In the Ordos Basin, one of the most important oil- and gas-bearing basins, the Triassic Yanchang Formation has formed important source rocks, but is also a typical representative of continental deep-water sedimentation. In the Huacheng area of the Upper Triassic Yanchang Formation, the deep-water gravity flow sedimentation characteristics of the lake-bottom fan are complex, and the spatial distribution pattern and stacking style of the sand bodies are of great significance for oil and gas resource exploration. Based on core observation, by combining well logging and analysis of signs of sedimentary facies, including petrologic features and primary sedimentary structures, the thick massive sand bodies of the Chang 6 Member belong to deep-water gravity flow deposits, and they develop in a semi-deep to deep lacustrine environment in the Huachi area, Ordos Basin. The primary sedimentary structures of deep-water gravity flows include massive bedding, graded bedding, sliding fractures, slumping deformation structures, turbidite sequences, and synsedimentary offsets. Two kinds of deep-water gravity flows of the channel system, namely sandy debris flows and turbidity currents, were identified in the sublacustrine fan. The sublacustrine provided accommodation space for the rapid unloading and accumulation of gravity flows. Deposited sandy debris flows are the most widely distributed in the sublacustrine fan. Three types of stacked sand bodies developed in the Chang 6 Member of the Huachi area, including multi-stacked thick-layered, sandstone–mudstone interbedded, and sand-thin and mud-thick types. The multi-stacked thick-layered sand bodies consist of multi-period massive sandstones, which are interpreted as sandy debris flow deposits. Sandstone–mudstone interbedded types exhibit diverse lithologies, including massive sandstone and deformed structural sandstone. In addition, the turbidity current is the primary factor controlling the stacked sand bodies. Sand-thin and mud-thick sand bodies consist primarily of laminated mudstone, massive mudstone, and flaser-bedded sandstone, and these deposits were formed by waning-stage turbidity currents and the rigid heads of sandy debris flows.