Study of the Development and Preservation of Lacustrine Beach and Bar Based on the Modern Sedimentary Characteristics of Qinghai Lake

Key words: 
• beach and bar /
• modern sedimentation /
• formation mechanism /
• preservation condition /
• Qinghai Lake

Abstract: Beach and bar sandbodies are an important research area in the petroleum exploration industry,since they are the result of repeated elutriation by wave action and are therefore favorable to the development of hydrocarbon reservoirs. A field investigation of modern sediments at Qinghai Lake (the largest inland lake in China)has established detailed identifying markers for beach and bar sandbodies from many aspects,such as sedimentary environment,hydrodynamics and sedimentation characteristics. Low-angle forewash cross-bedding,inversely graded (beach sand), normally graded (bar sand)and the absence of mud are typical for identifying signs. Further analysis showed that the material source and provenance,hydrodynamic forces,bottom morphology of the lake basin,lake shoreline and lake base level cycles are the main controlling factors marking the development,both formation and preservation,of beach and bar sandbodies. The provenance supplies the material base for beach and bar formation,and hydrodynamic forces boost sediment reshaping and beach and bar formation. The bottom morphology and shoreline of a lake basin determines its position in plan,and the extent of beach and bar formation;fluctuations in the lake base level determine the extent to which the beach and bar formations have been preserved. This study also points out that the formation of beach and bar sandbodies in a lacustrine basin occurs relatively easily,but their preservation is problematic. When suitable conditions of provenance,lake basin bottom morphology and hydrodynamics are present,low-stand system tracts,high-stand system tracts and lacustrine transgressive system tracts may form beach and bar sand deposits,but only the sand deposits at the early stage of the second order sequence,or at the lacustrine transgression phase of the third-order sequence,are well-preserved. Since beach and bar sand deposits are universal in continental lacustrine basins,this study has a particular reference significance for oil and gas reservoir lithology for exploration and development in continental basins in China.