The Structural Character And Tectonic Evolution Of Yitong Basin

Yitong basin is a Cenozoic petroliferous basin which develops in the northeast part of the Tan-Lu fault belt and locates between the Changchun city and Jilin city of Jilin province. The northwest edge of the Yitong basin is Daheishan horst which adjacent by the Soliao basin and the southeast edge is the Nadanhadaling terra. The basin's form looks like a long belt (140km long and 12～20km wide)and extend for the direction of NE45°,The basin's total area is about 2400km².On the base of the theories of geodynamics, strike-slip and extensional basin, inversion tectonics, petroleum geology and so on, this dissertation mainly discussed tectonic setting, structural characters, tectonic evolution and it's origin of Yitong graben, and analyzed the relationship between tectonic activities and oil-gas accumulation.Through the analyses of geodynamics setting, volcano activities, structural characters and evolution history of the structures around or inside Yitong basin.It shows the basin underwent various tectonic activities such as strike-slip, extension, compression, uplift and volcano activities in it's evolution history. The structure evolution profile indicates the Yitong basin's mainly period of tectonic evolution and the structural evolution mechanism, confirms that Yitong basin is a strike-slip and extensional basin which has a complex evolution history controlled by strike-slip faults system. The basin's evolution model can be divided into two stages: right-lateral extension and terra uplift period (middle Eocene to later Oligocene) and right-lateral extrusion and basin's base uplift activities period (Neogene to Quaternary).The evolution stage of Yitong basin include original faulted depression period (K-E₁) , faulted depression period (E₂s-E₂sh),stable subside period(E₂sh-E₂y),difference subside period(E₃w-E₃q),tectonic inversion period (E₃q-Nc) and bend depression period (Nc-Q) . This evolution process has great correlation with the area tectonic setting and the movement character of the Tanlu fault.The basement hypsography of Yitong basin is very complex and change evidence, basement faults extend in difference directions, the southeast part of the basement hypsography is high and the northwest part is low. The total conformation has the characters of "three apophysises, four concaves, two slopes and one thrust fault belt. Based on the basement hypsography, faults distributing character, stratum character, tectonic evolution and petroleum exploration actuality, Yitong basin has been divided into three second-distinction units of Moliqing faulted depression, Luxiang faulted depression and Chaluhe faulted depression. The third-distinction units include 14 units such as northwest boundary fault- folds belt, Kaoshan subsag, Jianshan structure belt, Wuxing structure belt and Wanchang structure belt, and the character, origin, petroleum conditions of the mainly structure units has been analyzed in detail.According to the analyses of faults' distinction, scale and their origin, this paper indicates that the fault characteristics is the mainly structure character in Yitong basin, the characters and evolution of the basin's faults are very complexity, the faults can be part into three distinction by their characters and sizes, the first distinction faults include the north-west boundary fault and the south-east boundary fault; the second distinction faults include the stretching faults such as Maanshan fault, No.2 fault, No.3 fault, No.4 fault and the thrust fault by the north-west edge; the third distinction faults include other faults of the whole basin. The mainly faults' character, origin and evolution has been analyzed in detail, especially the boundary fault in northwest margin of Yitong basin is quite different from that in southeast margin. The northwest boundary fault was a strike-slip fault and extend in beeline, which controls the basin's forming. It is a sinistral strike-slip and extensional fault in Eocene. In Neogene, it becomes a reverse fault under regional compression, which controlled the special structure belt's forming of the northwest boundary fault-folds belt. The southwestern boundary fault was an extensional fault and it extend in various direction, which served as an accommondating fault during basin's evolution.No.2 fault, No.3 fault, No.4 fault are the branching normal faults of southeast boundary fault. In Neogene, they suffer from compression and uplift.Through the analyses of the new data of seism, aeromagnetic, drilling, etc, this paper analyzes the character, origin and the significance of the inversion structures in Yitong basin. There is a great difference of the inversion structures between the northern-west edge and southern-east edge. In the northern-west edge, the tectonic stress shows compress, a fault- folds belt waged from 2 4km formed by the northern-west edge and it has good hydrocarbon pooling conditions, maybe it is a new area of oil and gas exploration, the shallow level of fault- folds and the footwall of thrust faults are important hydrocarbon exploration areas; in the southern-east edge, the tectonic stress shows uplift and compress, inversion structures such as extensional fault-uplifts mainly form at Jianshan and Wanchang structural belt, it destructs the preexisted hydrocarbon pool, the deep level and the slope of these structure belts such as Jianshan and Wanchang are important hydrocarbon exploration areas . The thrusting activity of the Tan-Lu fault zone since Neogene and the inversion structures of the Yitong basin take place in the regional dynamic setting that back-arc spreading caused by westward subduction of the Pacific plate, and the extrusion from the Indian plate collision plays a partial role in the E-W compression in East China. Besides this, the distribution and uplift strength of rigid plates also play a great role on the distribution and inversion strength of inversion structures of Yitong basin. Synthesize analysis indicates that Yitong basin's sediment character and oil-gas accumulation are mainly controlled by tectonic activities. Both in the right-lateral extension period (middle Eocene to later Oligocene) and right-lateral extrusion and uplift activities period (Neogene to Quaternary), basin's sediment system, thickness change of stratum and sediment center migration are all controlled by faults' strike-slip, tectonic subside and uplift. Middle Eocene to later Oligocene is the most important period for the basin's form, in this period the involution character of the Yitong part of the Tan-Lu fault zone shows the south part forms earlier than the north part and the fault's movement intension migrates northward through time, the sediment center of Yitong basin also migrates from southwest to northeast by the time and the sediment center close with the northwest boundary fault. Neogene to Quaternary is the basin's reversion period and the basin's two boundary faults begin right-lateral extrusion and uplift activities. In the southeast edge, the tectonic stress shows uplift and compress, inversion structures such as extensional fault-uplifts mainly form at Jianshan and Wanchang structural belt, it destructs the preexisted hydrocarbon pool; In the northwest edge, the tectonic stress shows compress, a fault-folds belt waged from 2 4km formed by the northern-west edge.Based on the tectonic analysis on Yitong basin together with other petroleum geology conditions, northwest boundary fault- folds belt is proposed to be the best fault tectonic zones benefit to oil-gas accumulation. It has good hydrocarbon pooling conditions and maybe is a new area of oil and gas exploration, the en-echelon anticline belts, en-echelon fault-block and the footwall of thrust faults formed by strike-slip-thrust movement are the important hydrocarbon exploration areas. Besides this, the slope area around structural belts (such as Liangjia and Wanchang structural belts) and ancient buried hills are also the benefit area for oil and gas exploration.