Study Of Sedimentary Characteristics Of Main Sand Bodys In Taiyuan Formation To Eighth Member Of Shahezi Formation In Daning-Jixian Area

The layers of Carboniferous and Permian,Ordos Basin,developed wide and thick coal seams with abundant natural gas resources.The Upper Paleozoic sandstone in Daning-Jixian area developed and have stable thickness.It is one of the most important production zones for natural gas exploration.In this paper,based on the previous studies,combined with large quantities of drilling,logging and geochemistry data by means of recognition of the lithology and logo layers,assisted with TEM to identify and divide the main sand groups of Taiyuani formation to the eighty member of Shihezi formation.Emphasis on the main layers with good development and large thickness:the second member of Taiyuan formation,the second member of Shanxi formation,the first member of Shanxi formation,the eighty member of Shihezi formation.Based on the previous studies,through trace elements analysis to reveal the changes of paleoclimate,paleosalinity,redox conditions and the operation mode of water.The results show that the climate of the upper Paleozoic began to turn cold since the Permian in the study area,sedimentary environment changed from reduction to oxidation,water salinity was transformed from brackish water to fresh water,the influence of the northern river supply was greater than that of the south.The sedimentary facies of Taiyuan formation to the eighty member of Shihezi formation in Daning-Jixian area are summed up in two major periods:Transition period of sea and land(Taiyuan formation),Continental sedimentary period(Shanxi formation to the eighty member of Shahezi formation).The depositional environment of the coal stratum(Taiyuan formation)was broad regression.Regressive direction from west to east.The coal bearing strata of Taiyuan formation developed shallow shelf facies,barrier island facies,tidal flat facies,lagoon facies.The sea water final withdrawal of research area at the end of the Taiyuan formation.The main sand body developed area is the barrier island sand and sand flat.Divided by Shallow shelf.Two developmental zones are formed in the study area,the sand mostly in the north-south direction.Period of Shanxi formation to the eighty member of Shihezi formation,with the supply of North and South,catchment developed in the study area.The sedimentary facies of underwater distributary channel facies,estuarine bar facies,inter channel facies and delta front mud were developed.It can be divided into delta front depositional system and pre delta-shore shallow lake depositional system,delta front sedimentary was main deposition system.Underwater distributary channel and estuary sand bar are mainly facies belts.The sand of estuarine dam shows east to West,The underwater distributary channel sand bodies are mostly affected by the direction of source supply,and are mostly distributed in the south-north direction.The upper Paleozoic sandstone reservoirs in the study area are mainly clastic reservoirs.The main reservoir space is secondary porosity with poor porosity and permeability.Because of the change of the material source,the structure of the lithology is changed vertically,the content of quartz is slightly reduced,the content of the debris increases,the volume of the filler and the cemented substance fluctuates little,and the total feldspar content is less.On the basis of summarizing the evolution rule of sedimentary facies and the statistics of main sandbody distribution and physical property,come to the conclusions that the Daning Jixian area sandbodies are mainly developed in the YH to DJ7-6 well area,the northesten parts of Yichuan area,JT4 well and JS5 well area.