Structural Features Of Permian-Triassic Unconformities In Luliang Uplift,Junggar Basin

Major hydrocarbon discoveries in Mahu depresssion confirm that the upper and lower unconformities of Permian-Triassic are important targets for hydrocarbon accumulation and exploration.The distribution of lithologic-stratigraphic reservoirs is dominated by large unconformities between the Permian and Triassic,P₂w and P₃w.Considering that the Junggar Basin from Late Permian to Triassic is a unified depression,and the tectonic and sedimentary characteristics of the eastern and western regions are similar,so the Permian-Triassic unconformities in the Luliang uplift of Junggar Basin are selected as the research objects.Preparing for the search for Permian-Triassic large-medium unconformity reservoirs.Basin characteristics and evolution process of Luliang uplift during Permian-Triassic period determined by regional tectonic study.Stress release after island arc collision forms Early Permian graben basin,then the Middle and Late Permian are transformed into the foreland basin.Triassic is a larger down-warped basin.Integrated stratigraphic interface features divide the Permian-Triassic into six sequences.The Permian and Triassic are divided into three sequences,PSQ1 is the Xia Zijie formation?Jiang Junmiao formation?,PSQ2 is the lower Wuerhe Formation?Ping Diquan formation?,PSQ3 is the upper Wuerhe formation?Wu Tonggou formation?,TSQ1 is the Bai Kouquan Formation?Upper Cangfanggou Group?,TSQ2 is the Karamay formation,TSQ3 is Bai Jiantan formation?Huang Shanjie formation?.Through well-tie sections and seismic profiles to complete stratigraphic correlation,find the upper Permian strata are absent in the west of Liliang uplift,the middle and lower Permian strata are absent in the east of Luliang uplift.Most of the study area is alluvial fan,fan delta,lacustrine deposits.Most of the Luliang uplift is fan delta deposit,and alluvial fan deposit exists locally.The fan delta front distributes in a large area,which is favorable for the development of migration and accumulation layers.The entire study area is an alluvial fan-fan delta-lake sedimentary system,most of the Luliang uplift belt is fan delta deposit,localized alluvial fan deposit,the fan delta front is widely distributed,which is favorable for the development of migration and accumulation layers.In this paper,a three-level division scheme of unconformity structure is proposed.The first level is caprock,unconformity layer and oil source network layer.The second level includes subdividing the unconformity layer into overlying layer,unconformity layer and underlying layer.The third level is the element level of reservoir formation,including migration and accumulation layer,barrier layer,weathered clay layer and weathered leaching zone.Vertical structure division of unconformities by taking logging data as the main method and combining lithological and physical characteristics.The paper considers that the tertiary structure of the study area is well developed,which can be divided into five unconformities and eight lithologic assemblages.A total of 16structural types of unconformities were classified.The paper describes the distribution characteristics of the migration and accumulation layer and barrier layer in the unconformity structure,the plane distribution characteristics of weathered clay layer and weathered leaching zone,and the distribution characteristics of the unconformity structure types.Finally,it is determined that the Dinan and Baijiahai-Wucaiwan are favorable areas for the formation of unconformity traps.Combining with the analysis of three typical unconformity reservoirs,the evaluation factors of unconformity reservoir formation are determined.Starting with the evaluation of oil and gas transmission system and unconformities,the favorable reservoir-forming areas of each unconformity structural layer are selected.The favorable areas of Ping Diquan formation are mainly distributed in the southern slope of Dinan bulge and the western slope of Xiayan bulge.Faults in the east of Luliang uplift play an important role in oil and gas transmission and reservoir formation.The tectonic background of gentle slope is a key element to west of Luliang uplift.Prediction of favorable reservoir-forming areas of Upper Permian and Lower Triassic mainly in the east of Luliang uplift.Combined with tectonic evolution and reservoir-forming stages,the favorable areas are mainly distributed in the Dinan and Baijiahai areas.The favorable zone of the Pingdiquan Formation is mainly distributed in the slope zone on the south side of the Diannan and the slope zone in the western section of the Xiayan bulge.The Upper Permian and Lower Triassic are mainly predicted by Ludong,combined with tectonic evolution and accumulation period.The favorable zones are mainly distributed in the Diannan bulge and the Baijiahai bulge.However,the Diannan bulge is divided into the favorable zone of the Permian source in the southern belt and the favorable source of the Carboniferous in the northern belt.The whole is closely related to the distribution and nature of the fault.However,the Dinan bulge can be divided into favorable areas of Permian source in the southern zone and favorable areas of Carboniferous source in the northern zone.The analysis of the paper shows that it is closely related to the distribution and nature of faults.