The origin and lithofacies development characteristics of fine particle composition in the shale of the second member of Fengcheng Formation in Mahu Sag

Shale oil, as one of the most important unconventional oil and gas resources, has become the key target of oil and gas exploration in recent years. The Fengcheng formation in Mahu Sag is the best source rock in the sag, which has great potential for shale oil resources and is the key area for shale oil exploration in Junggar Basin.

Volcanic activity was frequent during the sedimentary period of Fengcheng Formation in the northern part of Mahu Sag. The sediments are mainly composed of tuff material of volcanic activity, evaporation material of caustic lake and a small amount of detrital material. The terrigenous detrital material mainly comes from long-distance transport, while the pyroclastic material is closely related to the proximal volcanic activity. The lithofacies development of shale is characterized by frequent overlapping of various lithologies, diverse combination types and rapid changes. The microfabric of fine-grained sedimentary rocks is characterized, the lithofacies types of fine-grained sedimentary rocks are summarized, and the assemblage relationship and development law of lithofacies in different environments are analyzed. The formation process of lacustrine fine-grained sedimentary rocks is discussed from the perspective of provenance supply and sedimentary dynamics, and the lithofacies development model of fine-grained sedimentary rocks is established. To a certain extent, the theory of lacustrine sedimentology is enriched and perfected, and it can also provide basic geological basis for tight oil exploration in this area.