The Genesis of Sandbody in the Shallow Delta from Chang 8_1 of Jiyuan Area in Ordos Baisn

Chang 8member in the Ordos Basin belongs to shallow-water delta sedimentation,which is different from typical Gilbert delta deposit.Using data of logging,drill core and thin sections,this paper analyzes the transportation distance and the supply of provenances,the properties of the lake water and hydrodynamic condition of Chang 81sandbody in Jiyuan area,so as to determine its sedimentary environment and formation condition.The analysis suggests that the sandbody of Chang 81in Jiyuan area is close to the provenance and the provenances are enough and has stable supply ability.In the study area,Chang 81belongs to shallow-water delta deposits which are characterized by shallow and brackish water and gentle topography of the bottom of the lake basin.During the ascending period of base level of Chang 81the aggressive expansion of the water made the driving force of the water expansion greater than that of the river flowing into the lake.On the condition of enough and stable provenance supply and the driving force of the water expansion greater than that of the river flowing into the lake,the sandbody passivity unload ahead of time due to lake water strong top support action,thereby creating a continuous retrogradation sandbody with unobvious superposition stages in vertical but long extended distance and good continuity in horizontal.