Study On The Jurassic Prototype Basin In The Western Part Of The Northern Qaidam Basin

Jurassic is an important hydrocarbon source and coal strata in the northern margin of the Qaidam Basin.Due to the transformation and destruction of the complex tectonic movement,there is a lot of controversy about the Jurassic prototype basin and the original sedimentary facies in the northern margin of the Qaidam Basin,which has restricted the oil and gas exploration and the understanding of the basic geological evolution.Based on the previous research results,this paper,based on the research ideas of the reconstruction of the basin,was carried out by means of outcrop profile observation,seismic and drilling logging data analysis and multi-method provenance analysis.Stratigraphic division,sedimentary facies analysis,provenance comprehensive analysis and other research,made the following main understanding:Due to the influence of the later severe tectonic transformation,the Jurassic distribution in the study area is complicated,and it is overlapped and buried.The Lower Jurassic is mainly distributed in the south,middle and upper Jurassic of the Lenghu-Nanbaxian tectonic belt.The Mesozoic Jurassic distribution is consistent with the distribution of the Mesozoic.Through the outcrop and drilling sedimentary facies analysis,it is concluded that the Jurassic in the study area mainly develops the sedimentary facies types such as braided river,braided river delta,fan delta and lake.The large coal ditch mainly develops the braided river delta plain and the delta front subfacies,and the late development of semi-deep lake sedimentary facies,only a few outcrops or sections visible edge facies deposits.Based on the statistical analysis of the light and heavy minerals,cuttings,gravel components and debris zircon ages in the study area,it is concluded that the main provenance areas of the Early Jurassic in the western part of the study area are Altun Mountains,followed by the uplift of the northern margin.The Middle and Late Jurassic origin is mainly from the southern Qilian,followed by the northern margin of the local uplift.Based on the interpretation of seismic data and the sedimentary facies,combined with the regional geological data,it is concluded that the Jurassic basin in the northern margin of the Qaidam Basin has experienced three evolution stages:the early Jurassic early fault depression and depression,the early Jurassic mid-Jurassic extensional depression,the late Jurassic crished depression.The early Jurassic small faulted basins are mainly developed in the Qunteyi depression,and the depression basins are mainly developed in Yibei depression.The middle Jurassic depression basin is mainly developed in the Saishenteng-Yuqia depression and the Suganhu depression.The sedimentary range of the Late Jurassic extrusion depression is narrowed and migrated to the eastern part of the northern margin of the Qaidam Basin.Based on the analysis of the characteristics of comprehensive stratigraphic features,sedimentary facies,provenance characteristics and the uplift period of the surrounding mountain belt,the sedimentary boundary of the Early Jurassic primitive basin is roughly as follows:the west to the eastern margin of the Altun Mountains,the eastern and northern borders in the tectonic belt of Lenghu-nanbaxian;Southern boundary of the doubt.The southern boundary of the Middle Jurassic is as follows:the western boundary and the south boundary are in the south of the Lenghu-Nanbaxian tectonic belt to the south and the northern boundary in the southern margin of the Qilian Mountains,the eastern boundary is the easem of the Yuqia depression and the front of Liiliang mountain.