Fluid Transformation and Controlling Factors of the Fan Delta:A case study of the Xiligou delta in Qinghai
[Objective]Fan deltas are widely developed in the margins and interior of sedimentary basins,with a large scale,diverse types,and idea source-reservoir-caprock association,and have huge exploration potential for oil and gas resources.However,its fluid type transition and controlling factors,sedimentary process and characteristics of the fan deltas are still unclear.[Methods]Taking the Xiligou fan delta in Qinghai Province as the research object,based on the fine description of outcrop and trench profiles,the characteristics of grain size,structural characteris-tics of sediments in the Xiligou fan delta during different periods were studied by comprehensive use of 14C dating and grain size analysis.[Results and Conclusions]Through the fine description of the field outcrop trough section,9 types of lithofacies and 7 vertical combination sequences are divided,and 3 types of fluid types including debris flow,sheet flood and traction flow are identified.According to AMS14C dating,the Xiligou Lake delta is divided into three stages,SⅠ,SⅡ,and SⅢ by calibrating the characteristics of the sedimentary interface,sediment grain size,and structure.The SⅠ stage is a coarse-grained sedimentary stage of clastic flow,and the SⅡ stage is the deposition of sheet-flood reformation.Stage SⅢ is the period of traction flow reformation and deposition.Based on the analysis of sedimentary particle size-sensitive components,three lobe stages were identified in the delta plane of Xiligou Lake.A comprehensive analysis of the Xiligou delta slopes and the changes in the sporopollenin concentration in Qinghai Lake since 15 ka shows that the fuid type conversion of the Xiligou fan delta is controlled by the changes in the sedi-mentary slope and climate.The debris flow channel and sheet flooding sediments are the main architectural units.The lower plain of the fan delta was dominated by sheet flooding,and braided channels,braided bar,and flood-diffusion sediments were the main architectural units.The front of the fan delta was dominated by traction flow,and the main architectural units were subaqueous distributary channels,mouth bars,and sheet sands.
fan deltadepositional processfluid typeclimate changecontrolling factors
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