Sedimentary System And Gypsum Salt Distribution Of Wusongger Formation-awatage Formation In Tarim Basin

With the continuous exploration and research of oil and gas in Marine carbonate strata,Tarim Basin has always been the focus and hot area of scholars'research.The scholars'research on the source rocks and reservoirs in this basin is relatively rich,but the research on the middle and lower Cambrian cap rocks is less.Through outcrop geological survey,seismic interpretation,core observation and thin section identification,the author has revealed the transverse distribution and longitudinal development characteristics of the cap rock,as well as the main controlling factors of the cap rock development.Four tertiary sequence interfaces are identified in the Wusonger Formation and Awatage Formation,which are T₈⁴(bottom of Wusonger Formation),T₈³(bottom of Shaylik Formation),T₈²(bottom of Awatage Formation)and T₈¹(bottom of Lower Qiulitage Formation)from bottom to top.SQ1(Wusonger group)between T₈⁴-T₈³,SQ2(Shayrik group)between T₈³-T₈²,and SQ3(Awatag group)between T₈²-T₈¹.Among them,the lithology of Wusonger Formation is mainly argillaceous dolomite and dolomite.The lithology of Shailik Formation is dominated by arenaceous limestone.The lithology of Awatage Formation is mainly argillaceous dolomite,arenaceous dolomite,dolomite-bearing limestone and argillaceous siltstone,and also contains gypsum,halite crystal and algomite.Compared with Wusonger Formation,Awatage Formation has more paste and salt.The three third-order sequences can form a second-order sequence SSQ1(Early to Middle Cambrian)with only descending semi-cycle,which reveals the sea-level change process of"rapid transgression and slow regress".At the same time,two sequence correlation profiles are constructed to clarify the sequence filling pattern of"thick in northwest and thin in southeast"in the Middle and Lower Cambrian.The distribution law of seven sedimentary facies in Wusonger Formation-Awatage Formations is revealed.From SQ1(Wosonger Formation)to SQ3(Awatage Formation),the sedimentary pattern was"platform facies in the west and basin facies in the east".Three salt lake centers were developed in Awati sag,Bachu uplift and Tazhong uplift,and evaporative platform,restricted platform,open platform,platform margin,slope,shelf basin were developed in the periphery of Wusonger Formation.With the decrease of sea level,the range of evaporative platform,restricted platform and open platform becomes larger,and the range of shelf and basin facies in Awatage Formation becomes smaller.It reveals a Marine sedimentary model of"continental shelf facies developed in the north and south sides of the lower Cambrian and west taitung basin facies in the middle Cambrian".It is revealed that three polysalt centers developed in the lateral direction of Tazhong uplift,Awati sag and Bachu uplift in the Tarim Basin during the two periods of SQ1 and SQ3,and the distribution of gypsum cap rocks in the Awatage Formation is wider.Longitudinally,five sets of evaporite caps are developed,which are composed of five types of evaporite facies.The gypsum rock has the sedimentary characteristics of"thin distribution in Wusonger Formation,thick distribution in Awatage Formation"and"salt rock in the inner circle,gypsum rock in the middle circle and mudstone in the outer circle".The influence of extensional faults on the sealing ability of the cap rocks is analyzed.The dustpan-like depression of the Middle and Lower Cambrian directly affects the sedimentation,and the gypsum salt rocks are well developed where the extensional faults are developed.The development model of the middle and lower Cambrian thick evaporite cap rocks in the Tarim basin is established,that is,the dry and hot climate background,the continuous and slow regressive process,and the well-developed reef-shoal body as the barrier.