Cretaceous Sedimentary Filling And Fault Subsidence Control In Doseo Depression,Chad

Doseo Depression is located in South Chad Basin,central Africa,which belongs to "central rift basin group" of Central African rift system tectonically.It has experienced torsional rifting stage in early Cretaceous,compressional reversal stage in late Cretaceous and depression stage since Paleogene,and was mainly composed of a set of continental clastic rocks from Early Cretaceous to Holocene.The Upper Cretaceous was divided into Kome Formation and Miandoum Formation,which was denuded completely.The Lower Cretaceous was divided into Mangara Formation,Kedeni Formation,Doba Formation and Koumra Formation from bottom to top.The lower Cretaceous is well preserved in the eastern part of the Doseo Depression.The South Chad basin has been proved to be an oil-rich basin.The Doseo Depression has great potential for oil and gas resources and has made important discoveries.However,the oil and gas exploration is still in the initial stage,many sedimentary reservoir problems need to be solved.Therefore,on the basis of the studies of the regional geological background,guided by Vail’s classical sequence stratigraphy theory,this paper choosed lower cretaceous in the east of Doseo Depression as the research object and used drilling,logging and seismic data to build the sequence framework,analyze the depositional filling process,study the control effects of the fracture of deposition and provides feasible suggestions for further exploration.Through research,seven sequence boundaries are identified in the lower Cretaceous in the east of Doseo Depression and divided into six sequences from bottom to top: SQ1(Mangara＿low),SQ2(Mangara＿up),SQ3(Kedeni＿low),SQ4(Kedeni＿up),SQ5(Doba),SQ6(Koumra),among which the SQ3 and SQ4 sequences of Kedeni Formation are the key research stratas.From SQ1 to SQ5,the basin type changed from "narrow deep" type to "wide shallow" type with the expansion of the basin area.The meandering river delta was regressive,causing its thickness thinned,distribution area increased,and lake deposition increased.SQ5 to SQ6 sequences experienced structural inversion,lake basin area contracted,and the top of SQ6 sequence suffered denudation.Fan delta-braided river delta-meandering river delta-lake sedimentary system were developed in the depression.Fan delta is mainly distributed in the northern steep slope zone,with a small scale.Braided river delta and meandering river delta are mainly distributed in the southern gentle slope zone,with a wide range.The widely developed delta is dominated by underwater distributary channel sand body,and the delta framework sand body is filled with lacustrine sediments.The sedimentary distribution is controlled by ancient landform,ancient material source and fault activity.The overall shape of Doseo Depression is "double faults in the north and south,steep in the north and gentle in the South".In the early stage,the lake basin has small area,close provenance and intense fault activity,mainly alluvial fan and fan delta deposits.Differential settlement on both sides of the fault,and large difference in stratum thickness.In the later stage,the fault activity weakened,the stratum became gentle,the distributable area increased,and Braided river delta and meandering river delta developed.In addition,the sediments can extend along the depression trend in the middle of the fault zone.