Tectonic Characteristics And Its Controlling Effect On Oil And Gas Accumulation In Doseo Depression,South Chad Basin

The South Chad Basin is an important oil-gas bearing basin that has undergone multiple phases of tectonic evolution and has been a hot spot for oil and gas exploration in the world in recent years.The Doseo depression is located in the central part of the South Chad Basin,and up to now,the research on the Doseo depression,a depression dominated by strike-slip motion,is still weak.The study of the tectonic characteristics of the Doseo depression will not only help to understand the genesis mechanism of the Central African shear zone rift basin,but also better guide the oil and gas exploration.Following the principle of tectonic analysis,this paper analyzes the depression structure and tectonic style of Doseo depression by using 2D and 3D seismic data,recovers the depression tectonic evolution by equilibrium profile recovery method and extension of each sedimentary period,and analyzes the depression genesis mechanism with the regional geological background.Combined with the results of tectonic evolution analysis,the growth index and paleo-fault throw are calculated to quantify the fracture activity and reveal the characteristics and evolution history of the depression faults.Finally,the paper analyzes the types of tectonic traps through oil and gas geological data,and discuss the control role of depression structures on oil and gas from the perspective of fault lateral sealing ability.The Doseo depression is mainly filled with Precambrian basement,Cretaceous,Paleocene,Neogene and Quaternary,the main body is Lower Cretaceous,and the postPaleocene sedimentary stratigraphy is thin.The main body of the depression has an obvious double-layered structure,which is a deep fault structure superimposed on a shallow argillic structure.The tectonic styles include fault-folding structure,(antiformal)negative flower structure,fault terrace,graben-horst system and domino structure.The depression mainly underwent six phases of tectonic evolutionary activity.In the early Early Cretaceous,the depression was tensioned to form the initial rift valley.In the late Early Cretaceous,the depression was in a right-sided tense-torsional slip stress field and a central slip fault zone was formed.From the Late Cretaceous,the depression settled steadily until the occurrencee of the Santonian extrusion event in the Late Cretaceous,when the depression was reversed by extrusion and the depression structure was changed.In the Paleocene Eocene,the change of stress field caused compressional-torsional slip motion in the depression,which further modified the depression and formed new fractures.After the end of the Eocene inversion period,the tectonic movement of Doseo depression basically stopped and the strata settled steadily.The two phases of slip movement experienced by the depression have played an important role in influencing the depression structure.The two phases of slip movement are opposite in nature,with intensity ranging from strong to weak,with the early strong tension-torsional slip developing slip fractures in the central part of the depression,and the later less active compression-torsional slip mainly forming folds and some reverse faults.The tectonics provides hydrocarbon traps for the Doseo depression,mainly developing anticline,faulted anticline,fault nose and fault block traps.The trapcontrolling faults are connected in segments during the early extensional period,and the traps are finally defined during the Paleocene reversal.Reservoir formation in depression is mainly controlled by lateral sealing property of fault and sealing conditions.