Study On Sedimentary Characterstics Of Jurassic In Wu Dun Sag Of Dunhuang Basin

Dunhuang basin is located on the western end of Hexi Corridor,the structure belongs to the Tarim plate-the Dunhuang block,Wudun sag is a newly discovered Jurassic oil-bearing depression in Hexi Corridor,which lies in the east-central of Andun depression of Dunhuang basin.both of the wells Xican 1 and Dun 1 wells get high quality source rocks and have good shows of oil and gas in the district,at the same time,they producing low production industrial oil flow.It indicates that the Jurassic in the Wudun sag has favorable hydrocarbon accumulation conditions and exploration prospects.Due to the complex geological conditions,low degree of exploration and lack of information,the study of the sedimentary system in Wudun sag is coarse,and the sedimentary characteristics of the main target intervals are not certain,which seriously restricted the exploration progress.Therefore,based on the theory of sequence stratigraphy and seismic sedimentology,this paper studies the Jurassic sedimentary facies of the Wudun sag,which is great significance to clarify the exploration direction of the next step in the Wudun sag.In this paper,the sequence stratigraphy and sedimentary characteristics of the Jurassic in the Wudun sag are studied by using the outcrops,drilling,logging and seismic data in the Wudun sag.1.According to the analysis of outcrop,drilling and seismic data,it is considered that there are two main source areas in the Wudun sag,which are the northern mountain and the sanwei mountain in the southern.The Sanwei mountain provide coarse sediment to the north,the northern mountain provide relatively fine-grained sediment to south.2.According to the seismic reflection termination relationship and logging data,the middle and lower Jurassic in the Wudun sag is divided into two third-order sequences,that are SQI and SQII,where SQI corresponds to the Lower Jurassic Dashankou Formation,SQII corresponds to the Middle Jurassic Both of the sequences can divide into three parts,those are LST,TST and HST.and the corresponding sequence stratigraphic framework can be established.3.Based on the stratigraphic division,the braided river,the sedimentary river delta,fan delta and lacustrine sedimentary facies of the tributary river delta are mainly determined by detailed core description and logging facies analysis.4.Wudun sag performance a fault depression control by the Sanwei mountain fault.during the deposition period of the Dashankou Formation and Zhongjiangou Formation,the Wudun sag is a faulted depression which is distributed along the Sanwei mountains,during the sedimentary period of the middle Jurrasic,Which is a wide and shallow lake basin.During the Late Jurassic period,the whole basin entered into depressing stage.5.Based on the results of provenance source analysis,outcrop phase,single well facies and seismic facies analysis,the sedimentary facies model of the Wudun sag is established,and the evolution of the Early Middle Jurassic sediments in the Wudun sag is clear,and the fan delta deposition System,the northern and northeastern regions of the braided river delta deposition system,depression central depression for the lake sedimentation.In the early Jurassic period,the fault is characterized by obvious fault,the slope is large,the deposition rate is fast,and the rapid accumulation is mainly in the nearshore.Early from the early Jurassic to the early Middle Jurassic,the depression continued which is characterized by warm and humid climate,sufficient supply of sag source,high-level braided river delta deposition system in northern slope zone,and subsoil channel sandbody is a good reservoir.Layer,the southern fan delta is also favorable sand body development area.