Abstract:Hybrid event beds are common in lacustrine basin, their formation and distribution is significance for understanding the evolution processes of gravity-flow and the conventional and unconventional oil and gas exploration and development in their deposits. Taking the gravity-flow deposits in the first member of the Liushagang formation in the Weixinan sag and the seventh member of Yanchang formation in the Ordos basin as examples, the sedimentary characteristics, types, formation mechanisms, depositional model as well as the oil and gas geological significances of hybrid event beds in lacustrine basin are discussed. The gravity-flow deposits in lacustrine basin including slide and slump deposits, sandy and muddy debirs flow deposits, high and low density turbidity-current deposits, and the common hybrid event beds. Three types of hybrid event beds are subdivided, which include sandwich structure, bipartite structure, and banded structure. The bipartite structured hybrid event beds can further subdivided based on the thickness difference between the upper and lower division. Sandwich structured hybrid event beds are mainly caused by flow erosion or liquefaction and located in the proximal of hybrid event beds. Bipartite structured and banded structured hybrid event beds, which located in the distal of hybrid event beds, are mainly attributed to flow decelerate and expand, the different settling of detrital grains in muddy debris flow. The vertical stacking of different beds with the same internal depositional unit composition in core is the reliable identification mark for hybrid event beds. If we couldn’t reveal the formation processes of these hybrid event bed, it may cause misunderstanding of the depositional information. At the same time, the development of hybrid event bed will add the depositional heterogeneity of gravity-flow deposits, which is bad for conventional oil and gas preservation. However, the fine-grained deposits caused by hybrid event beds are the good lithofacies associations, which is the sweet spot for unconventional oil and gas development.
