Sedimentary Characteristics And Evolution Of The Paleogene In The Yiliping Area,Qaidam Basin

The Qaidam Basin is located at the northeastern margin of the Tibetan Plateau and is the largest Meso-Cenozoic terrestrial petroliferous basin in the plateau.The Paleogene in Yiliping area shows a good hydrocarbon show,with a certain potential for oil and gas exploration.Limited to the Paleogene burial depth in this area,the exploration maturity is low,and no industrial breakthrough has been achieved yet.At present,the study on the division and correlation,provenance characteristics and the depositional systems of the Paleogene strata in the Yiliping area is relatively insufficient.Integrated the drilling data,well logging data,seismic data,core observation and sample analysis in this area,the thick strata of the Upper Xiaganchaigou Formation and Shangganchaigou Formation were subdivided,which are focused research of the Paleogene,the distribution and evolution of the Paleogene depositional system in this area are characterized and the distribution of favorable reservoir and seal assemblage is predicted,and provides a geological basis for oil and gas exploration in this area.According to lithology changes,sedimentary cycles,marker layers,etc,the Upper Xiaganchaigou Formation is divided into three submembers and Shangganchaigou Formation is divided into two members.The Upper Xiaganchaigou Formation is oxidized,and the second submember has the finest lithology and the lowest sand content.The lower member of the Shangganchaigou Formation has a high sand content rate,andr is an oxidized color.The lithology of the upper member gradually becomes finer and the color gradually reduces.Based on analysis of core observation,cutting logging,well logging and seismic data,it is believed that there are five types of sedimentary facies in the Paleogene in Yiliping area: alluvial fan,braided river,meandering river,delta and lacustrine.Analysis of lithology,chloride ions,carbon and oxygen isotopes believes that the Paleogene climate in the study area is generally arid,and the early-Middle Eocene climate was relatively arid,and the middle-late Eocene climate was gradually humid,and the late From the Eocene to the Oligocene,the climate became arid.Analysis of paleocurrents,light and heavy minerals,and seismic data indicate that the Paleogene provenances in the Yiliping area were mainly from the eastern Kunlun,Altyn Mountains,and southern Qilian regions.Based on sedimentology and provenance analysis,the Paleogene in Yiliping area was dominated by the shore-shallow lacustrine facies and the depocenter of the Yiliping area was located in the Hongsanhan No.3 and No.4 areas during the sedimentary period of the Upper Xiaganchaigou Formation and Shangganchaigou Formation.At this time,it was seperated from the depocenter in Western Qaidam Basin,whereas they were still in the same lake basin.The second submember of the upper Xiaganchaigou Formation was deposited in a relatively humid climate condition.Large-scale lake transgession occurred,with the largest lake basin.The depocenter was located in the Hongsanhan No.3 and No.4 areas.During the sedimentary period of the upper member of the Shangganchaigou Formation,the depocenter of the Yiliping began to migrate eastward.Based on the understanding of the tectonic evolution of the peripheral orogenic belt and the analysis of the Paleogene lithology,ZTR index,sedimentary rate and climate change in the Yiliping area,the source to sink evolution of the Paleogene in this area is mainly divided into three stages:(1)Early-Middle Eocene,the eastern Kunlun,Altyn Mountains,and southern Qilian are strongly uplifted and settled with red coarse detrital sediments recorded by the Lulehe Formation.The low ZTR index correspond to the long-term response caused by the collision between the Indian plate and the Eurasian plate in the early Cenozoic.(2)MiddleLate Eocene,the tectonic activities in the three source areas have weakened,corresponding to the high ZTR value recorded in the Xiaganchaigou Formation in the Yiliping area,and a large-scale flood plain-lake fine-grained sediment has been dominated.The deposition rate was gradually increasing,controlled by tectonic activity and relatively humid climate.(3)Late Eocene-Early Miocene,the tectonic activities of three major orogenic belts intensified,and the Shangganchaigou Formation in the study area recorded the coarse detrital sediments and low ZTR index.The Altyn Mountains uplift was the strongest,and there are obvious growth strata in the front of the mountain,which are angular unconformity to the overlying Xiayoushashan Formation,resulting in the depocenters of Western Qaidam Basin and Yiliping began to move eastward during the sedimentary period of the upper member of the Shangganchaigou Formation.The source rocks of the Shangganchaigou Formation in the Eboshan area in the southern Yiliping are of good quality.Hongsanhan No.3 and No.4 are the depocenters during the sedimentary period of the Upper Xiaganchaigou Formation and Shangganchaigou Formation in the Yiliping area.They can serve as high-quality source rocks and have good hydrocarbon generation potential.The nearby delta sand body reservoirs in Luoyan Mountain,Eboliang III,Yahu area and the delta front and plain mudstone caprock can constitute favorable reservoircaprock assemblages and have good prospects for exploration.