Key words: 
• beach bar sand body /
• cumulative probability curve of grain size /
• environmentally sensitive grain size components /
• Qigequan Formation /
• Tainan area /
• Qaidam Basin

Abstract: Abstract : In order to deeply evaluate the natural gas exploration prospect and predict the favorable exploration zone and exploration target of the shore-shallow lake beach-bar sand body of the Quaternary Qigequan Formation in Tainan area, eastern Qaidam Basin, based on the comprehensive analysis of core observation and drilling ( logging ) data, combined with the analysis results of rock thin section, grain size-standard deviation, grain size cumulative probability curve, environmental sensitive grain size composition and other parameters, the grain size variation characteristics and hydrodynamic significance of the shore-shallow lake beach-bar sand body of Qigequan Formation in this area were studied in detail. The results show that : First, The sedimentary period of Qigequan in Tainan area is the sedimentary environment of shore-shallow lake. The beach-bar sand body is developed, the rock is loose, the cementation is poor, easy to break, and the grain size is fine. The lithology is mainly lithic feldspar fine sandstone-siltstone, the composition maturity is medium-poor, and the sorting roundness is medium-poor. Second,During the sedimentary period of Qigequan Formation, the cumulative probability curve of grain size of beach-bar sand bodies in the study area showed six patterns, mainly one-hop one-suspension type ( 64.4% ), followed by multi-segment type ( 11.9% ), and the third is two-hop one-suspension type ( 7.5% ). Third, During the sedimentary period of Qigequan Formation, the grain size-standard deviation diagram of beach-bar sand bodies in the study area showed four patterns, mainly multimodal ( 50.0% ), followed by bimodal ( 33.4% ), unimodal and trimodal ( both 8.3% ). Combined with the cumulative curve of grain size probability and paleogeomorphology analysis, four environmentally sensitive grain size components were determined, which represented suspension transport, wave, coastal current and storm wave, respectively. Among them, wave was the main hydrodynamic type controlling the formation and development of beach-bar sand bodies, and the average value of environmentally sensitive grain size components accounted for 27.9% of the total grain size. Followed by the suspension effect ( average of 15.3% ) and the coastal current ( average of 11.4% ), the storm wave effect is the weakest ( average of 2.3% ). Forth, Through the study of the plane distribution characteristics of the percentage content of environmentally sensitive grain size components, it is shown that the influence of waves on the formation and development of beach-bar sand bodies is weakened from the core to the wing of the anticline, while the suspension effect is just the opposite, and the influence of coastal flow on beach-bar sand bodies is concentrated on the east and west wings of the anticline.