Provenance And Sedimentary Characteristics Of Shan-1 And He-8 Segments In The Middle Part Of Tianhuan Depression

Middle part of Tianhuan Depression of Ordos Basin is one of the important areas for oil and gas exploration.However,the research degree of this region is still relatively low,which restricted the deployment of the following prospecting work.Thus,this thesis,aiming at determining the evolution rules of sedimentary facies,carries out the research on provenance and sedimentary characteristics utilizing cores,well logging data,mud logging data as well as observation of outcrops.Results of the research can be provided as a basic for reservoir prediction.Using comprehensive methods of provenance such as analysis of detrital composition of sandstone,lithic fragment composition of sandstone,lithic fragment types of conglomerate,heavy mineral assemblages,cathodoluminescence image of quartz and sand-mud ratio,three source directions are considered in the research area during the period of Shan-1 segment and He-8 segment.These three provenance regions are Yinshan old land in northern Ordos Basin,Alxa old land in north-western Ordos Basin and Qilian–Liupanshan old land in the south-western edge of Ordos Basin.Provenance of Shan-1 segment and He-8 segment in the northern part of the research area is characterized as inheritance.From the beginning of He-8 segment,provenance in the southwest of Ordos Basin starts to provide sediments to the research area.Guided by the theory of high-resolution sequence stratigraphy,the sequence stratigraphic framework of research area is built up.Shan-1 segment and He-8segment are divided into three long-term cycles,seven medium-term cycles and a dozen cycles.Shan-1 segment and He-8 segment are divided into seven layers from the bottom to the top by medium-term cycles.They are Shan-1~3,Shan-1~2,Shan-1~1,He-8xia~2,He-8xia~1,He-8shang~2 and He-8shang~1.On the basis of color analysis of mudstone,observation of outcrops sedimentary characteristics,core identification of sedimentary facies and sequence stratigraphic classification,well-tie sedimentary facies correlation and analysis of plane facies have been done.The results suggest that depositional environment transforms from meandering river delta of Shan-1 segment into braided river delta of He-8 segment under the setting of lake-basin.Four types of micro-facies develop in Shan-1 segment.They are underwater distributary channels,subaqueous levee,tributary bay and mouth bar and the former two types are most developed.Underwater distributary channels mainly distribute in the lower middle part of Shan-1 segment.Six types of micro-facies are identified in He-8 segment,including braided channels,flood plain,overbank deposits,underwater braided channels,underwater distributary interchannels and mouth bar.Braided river delta front subfacies are most developed in lower He-8 segment,and braided river delta plain subfacies are most developed in upper He-8 segment.Channel scale is getting smaller from lower He-8 segment to upper He-8 segment.From the early period to latter period of Shan-1 segment,the distribution range of lake is getting more expanded and reaches to the maximum in the latter Shan-1period,but the channel scale is becoming smaller.During the period of He-8 segment,the range of the lake in the research area decreased first and then increased.The most favorable horizon for reservoirs is lower He-8 segment,upper He-8 segment is the next and the relatively bad is Shan-1 segment.