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

The Yiliping area is located in the central of the Qaidam Basin,and accepted a huge thickness of the Neogene strata.Different levels of hydrocarbon shows have been encountered in the Neogene,which indicates that the there certain hydrocarbon generation potential and reservoir-forming conditions.However,the development of the Neogene strata,the distribution of sedimentary facies,and the effective reservoir-caprock assemblage are not clear.In this study,the existing exploration data was fully utilized to divide and compare the Neogene in this area.On this basis,the distribution of sedimentary facies in key horizons was depicted,which reveals the changes of the paleoclimate and paleoenvironment of the Neogene The main source direction is clarified,and the “source to sink “response relationship is established.The distribution of favorable reservoir-cap assemblage in the study area is predicted,which will provide geological basis for the next oil and gas exploration.Based on the lithology,color,sedimentary cycle,etc.,the Neogene strata in the study area are divided and correlated in sections.The lower Youshashan Formation is divided into three sections,the upper Youshashan Formation is divided into two sections,and the Shizigou Formation is also divided into two sections.The stratigraphic correlation shows that the thickness of the Lower and Upper Youshashan formations of the Neogene in Yiliping area is thin in the south and thick in the north,and thick in the west and thick in the east,while the Shizigou Formation are thick in the east and thin in the west,reflecting that the subsidence center is basically moving from west to east regularly,the lithology generally becomes thinner from west to east and from the piedmont to the basin.The lithology of the first to third sections of the Lower Youshashan formation has changed from coarse to fine,and the color has changed from oxidation to reduction,and the content of lacustrine mudstone has gradually increased.The lithology of the first to second sections of the Upper Youshashan yormation changes from coarse to thin,and the color changes from red-gray to gray,and the whole is slightly reduced.The lithological changes of the first to second sections of the Shizigou Formation are zonal.From the bottom to the top,the western region shows coarse oxidation and the eastern region shows thinning and reduction.According to core observation and logging data analysis,it is believed that the Neogene in Yiliping and its surrounding areas are mainly developed: four types of sedimentary facies:alluvial fan,river facies,braided river delta,and lake.Ahloride ion concentration and carbonate content shows that the Yiliping Neogene period from early to late,the climate gradually became arid,the whole is dominated by arid and oxidizing environment,it is a lake basin that is constantly saline and shrinking.By using comprehensive analysis of light mineral composition,trace elements and heavy mineral combinations,it is believed that the Neogene sediments in the Yiliping area mainly come from the three directions of Altun Mountains,East Kunlun and South Qilian.There are differences in rock types,but they are mainly dominated by felsic and acidic igneous rocks.Based on the comparison of the development of the piedmont strata around the Qaidam Basin and the study of sedimentary records such as stratigraphic deposition rate,ZTR change,and climate change,combined with previous knowledge of the uplift time limit of the peripheral mountain range,the structure of the peripheral orogenic belt is considered The evolution has a good source-to-sink response relationship with the Neogene stratigraphic deposition records in the Yiliping area.During the early-mid Miocene(22~15.8Ma),there was a first-stage tectonic uplift event in the peripheral orogenic belts.All three provenance areas provide provenances the Yiliping area,and the deposition rate in the basin was high.From the late Miocene to the early Pliocene(12~7.2Ma),intensive uplifts occurred in the Altun Mountains and southern Qilian,but the sedimentation rate in the basins of the Altyn Mountains was low,presumably because of the climate in the northern region during this period The drought caused the Altyn Mountains to transport less material into the basin.Since the Pliocene(<5.1Ma),there has been a intensive uplift in the eastern Kunlun,which has increased the supply of sendiments to the Yiliping area,and close to the South Qilian and Altun Mountains.The Neogene in Yiliping hinterland mainly developed lacustrine facies,the alluvial fan-braided river-braided river delta system was developed in the north of the Altyn Mountains,the river facies-braided river delta deposition system was developed in the south of the east Kunlun Mountains,and the north The marginal piedmont is dominated by alluvial fan-river flood plain depositional system.There are two sedimentary centers in the Neogene in the study area,but the lake basin range and the size of the sedimentary center are different in each period.From the early to the late Neogene,the sedimentary centers have a tendency to migrate from west to east.The Neogene in the Yiliping area has TOC,and the source rocks are mainly developed in the bottom formations of the Lower Youshashan Formation and Upper Youshashan Formation in the Eboshan-Luoyanshan area,forming a favorable source-reservoir combination with the Neogene delta sand bodies which adjacent to them.The lacustrine mudstone developed in the upper part can be served as the cap rock of the reservoir,they all can form a favorable association which belongs to type of the self-generated and self-capping.