Sedimentary Characteristics And Architecture Research Of Sandy-gravel Beach-bar In Qinghai Lake

Lacustrine beach-bar sand-body has the characteristics of being close to the oil source,low mud contain,good sorting and permeability,it's an important target for oil and gas exploration.Modern sedimentary investigation is conducive to clarifying the specific formation process and sedimentary characteristics of the beach-bar,and it is the most direct and effective method for understanding and studying the beach-bar.Beach-bar sand-body is widely developed in Qinghai Lake,which is a favorable place to study the deposition of the beach-bar.Comprehensively utilized research methods including modern sedimentary investigation,satellite map observation,exploration trench,profile observation,particle size analysis,thin section analysis,scanning electron microscopy,etc.,the author studied the sedimentary characteristics and sedimentary model of the sandy-gravel beach-bar around Qinghai Lake.The specific sedimentary characteristics of sand-gravel beach-bar are summarized,and the sedimentary model of Qinghai Lake sand-gravel beach-bar is established by combining hydrodynamic analysis.On this basis,the research on the architecture of the beach-bar was carried out,and finally the identification mark of the sandy-gravel beach bar was proposed comprehensively.The sedimentary laminas of the beach-bar are always inclined to the lake,and only the overwash deposits which across the bar on the top can be tilted at a high angle to the land.Overwash deposits is of great significance in the sedimentary environment of beach-bar.The wave zone of Qinghai Lake can be divided into six parts: the symmetrical wave belt,the inclined wave belt,the wave breaking belt,the wave reborn belt,the secondary wave breaking belt,and the surfing-washing belt.Among them,type A sediments which is mixed by sand and gravel often develop in the inclined wave belt and the wave reborn belt;type B sediments which is massive gravel often develop in wave breaking belt and the secondary wave breaking belt;type C sediments which contain normal grading deposit often develop in the surfing-washing belt.And ABC sedimentary sequence of beach-bar is formed by these.A total of five microfacies are developed in the sandy-gravel beach-bar sedimentary environment in Qinghai Lake,which are adjacent to the semi-deep lake microfacies on the lake side.The order from the lake to the land is: offshore bar microfacies,inter-bar microfacies,coastal bar microfacies,overwash microfacies and lagoon microfacies.The single bar stacked pattern can be divided into four categories: transgression-side stacking,regression-side stacking,vertical stacking,and isolated pattern.There are two stacking patterns of beach-bar accretions: retrogradation-coverage and progradation-migration.The four key identification marks between the beach-bar and the mouth bar: abrupt contact of the lithologic top and bottom interface,developed ABC sedimentary sequence,undeveloped trough cross-bedding,and the elongate direction is always parallel to the lakeshore.