Sedimentary System and Evolution of Liushagang Formation,11-7 Block of Weixinan Depression

The deposition system of Liushagang formation is various.The study of its composition,configuration and evolution characteristics can provide guidance for deep exploration and development.Based on the core,well logging,logging and the testing data,we analyze the distribution of sedimentary systems of Liushagang formation in weizhou 11-7 area.The lithology of L3 member is coarse granularity including sandy conglomerate and pebbly sandstone,with bad sorting and rounding.Mudstone with oxide color shows shallow lake sedimentary environment and sedimentary structure and granularity material all showed the existance of debris flow deposition.All the characteristics indicate fan delta sedimentary system developes with rapid deposition.The lithology of L2 member composes a set of thick mud which reflects fairly deep-deep lake sedimentary environment.The major sedimentary system of L1 member is braided river delta and the lithology of L1 member is slightly better than that of L3 member.The hydrodynamic force of underwater distributary channel of delta front is strong and there are tractive current sedimentary structures,such as tabular cross-bedding,trough cross bedding,parallel bedding and wave bedding.Morever,there is more gravity flow and turbidity current in the area between delta front and deep lake.During the sedimentary period of Liushagang formation,the lake basin experienced a process of expansion and prosperity to shrink.The sedimentary stage of L3 member is the expansion period of lake basin.The heyday of lake basin is L2 member sedimentary stage,and deposit a set of thick mudstone.After the L2 member sedimentary stage,the lake basin begins to shrink and.During the evolution process,the sedimentary system gradually evolved from fan delta to fairly deep lake and braided river delta.