| Longmenshan orogenic belt is located in the west side of the Yangtze platform,Longmenshan orogenic belt from the northwest to southeast has four main fault zone:F1:Maoxian-Wenchuan fault(the mountain behind fault),F2:Beichuan-Yingxiu fault(the center fault),F3:Anxian-Guanxian fault(the mountain front fault)and Guangyuan-Dayi blind fault(the piedmont fault).In the tectonic trend,the Longmenshan orogenic belt is divided into three sections from the northeast to the southwest,namely,the north segment,the middle segment and the south segment,by the Beichuan-Anxian line and the Wolong-Huaiyuan line.In terms of oil and gas,5industrial gas wells were acquired in the Zhongba-Shuangyushi area in the north section of the Longmenshan,with the average test output per well reaching 566700cubic meters/day.Bitumen is widely distributed in the fault belt of the southern Longmenshan section,and the current proven reserves of natural gas in the Pingluoba gas filed are 165.35×10~8m~3.Therefore,Longmenshan-foreland basin is one of the most important gas exploration blocks in Sichuan basin.Paper put Longmenshan fold thrust belts-western Sichuan foreland basin system superficial tectonic erosion process as the research object,based on the basic theory of structural geology and petroleum geology,using the surface geology-deep geology,structural model and physical model to reveal the Longmenshan-western Sichuan foreland basin system superficial tectonic erosion how to control the system on the formation and evolution of basin-mountain structure in the late and middle Cenozoic,further,based on the basis of oil and gas geology to discuss petroleum geological significance.In practical work,on the basis of Longmenshan-western Sichuan foreland basin regional tectonic background,we have collected predecessors'research results,and carried out the field outcrop exploration,and geophysical interpretation of related research,preliminary revealed progressive Longmenshan tectonic evolution sequence is performed by multiple times,from Neoproterozoic to Cenozoic,tectonic activity from strong to weak to strong,the stress direction of the northwest-southwest-oriented and south-north-oriented sinistral thrust shear become dextral strike-slip associated brittle deformation,these features indicate that longmen mountain experienced at least two extrusion events.Based on the geological structure of longmenshan area in wetern Sichuan,in order to compare with the Longmenshan-western Sichuan foreland basin system,reveal the controlling effect of superficial tectonic erosion on the formation and evolution of the basin-mountain.Based on the critical wedge theory of fold and thrust belt and the principle of self-similarity growth process,the superficial structure erosion process is studied by controlling the different critical wedge angle in the physical model of sand box.First of all,we design the 6 groups respectively about standard experiments of erosion or deposition,through the rates of denudation and starvation or saturation deposition to compare the experiments,and the whole process of the experiments carried on the quantitative analysis.Both erosion and deposition can inhibit the development of wedge.Based on the interpretation of seismic data of Longmenshan-foreland basin and the actual thickness of strata,we designed 4 groups of actual proportion models.The falt top type erosion had obvious bedding decollement,formed a typical double structure,the critical wedging angle erosion formed a typical thrust structure.Then,we used glass beads instead of the silicone,and thickened the slip layer,revealed the different thickness of the slip layer mainly reflected in the wedge length increased,thrust distance increased and back-thrust didn't develop.In the comparative analysis with the deformation characteristics of Longmenshan-foreland basin,it is found that there is a strong coupling between the experiments.In addition,in terms of hydrocarbon geology,due to the control of the progressive thrust deformation process,different parts of the system have different tectonic deformation strength and uplift and erosion,and would inevitably cause the difference of petroleum preservation condition.Therefore,whether the effective caprocks in the tectonic deformation area of the Longmenshan-western Sichuan foreland basin have been destroyed is the critical factor for the good preservation of petroleum reservoirs. |
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