Study On The Pattern Of Thrust Structure And Oil&Gas Distribution In The Typical Piedmont Thrust Belt In The Central And Western China

The unique and complex deformation structure patterns and hydrocarbon accumulation characteristics of the large extrusion basin in central and Western China,which is the products of the dynamics of tectonic deformation in composite continents the complex.It is different from the related structure and hydrocarbon response under the typical plate system.The piedmont thrust belt under the background of plate system is characterized by thin-skinned structure.However,The thick skinned structural involved in crystalline basement of the piedmont thrust belt under the background of the internal environment in the central and Western China.It is obviously difficult to meet the needs of the study of the complex continental dynamics by the traditional method of dividing the geological structure of thick-skinned and thin-skinned.The conversion relationship and type difference between the two different structural belts and the control of oil & gas accumulation urgently need to be deepened.The results obtained will be of great significance to the theoretical study of the complex continental dynamics and the oil and gas exploration in the piedmont thrust belts.And,It is also helpful to further clarify the structure of the tectonic deformation in the piedmont thrust belts and its control effect on the accumulation and accumulation of oil & gas.So as to improve the prediction level of oil and gas distribution.The paper research object mainly focuses on the piedmont thrust belts of the large basin in the central and Western China.Mainly uses theoretical or technical methods,such as tectonic geology,geophysics,integrated geological analysis,and structural physical simulation experiments.A detailed study of typical multiscale geophysical profiles,field outcrops and oil and gas distribution characteristics."Deformation mechanism-structural classification-oil and gas response" as the main line of research.Based on the differences between typical piedmont thrust types and deep geological structure and tectonic evolution background,the deep dynamic mechanism of shallow crust structural deformation is analyzed,and the classification model of composite continental thrust deformation structure is established.On this basis,the research carried out the classification of structural deformation of typical piedmont thrust belts in center and Western China,combined with the size distribution of oil & gas,to analyze the relationship between the structural mode difference of the deformation zone and the current oil & gas distribution,to reveal the main controlling factors of oil & gas accumulation in the deformation zone.The results of the above study may guide the prediction of oil & gas distribution in the pattern of the deformed structure in the piedmont thrust belts.The deep geological structure of the large basin and the piedmont belt in the central and western regions has different characteristics in the geophysical field,the geological structure and the evolution of the thrust time sequence.The large basins in the central and western regions are located in the normal crustal and lithosphere areas.While,the surrounding orogenic belts in the large basin are in the Moho depth gradient zone.The bottom boundary of the lithosphere is characterized by ups and downs and local upwelling.There are different rheological properties of the deep lithosphere in the typical piedmont thrust belts of the central and Western China.Dabashan,Xuefengshan,Liupangshan thrust belts have a stable structure in vertical direction,and a high degree of coupling in the crust of the crust,and almost no crustal lateral shortening the formation of the "root".While,Longmenshan and both sides of Tianshan thrust belts have a relatively complexed structure in vertical direction,Discontinuity and discontinuity of deep seismic reflection interface,plastic layer development in the shell,and obvious “root”.The thesis makes a new interpretation of the main geophysical profiles across the thrust belts with "root".It shows that "rigid" high density lithosphere in the basin is subducted to the earth crust or lithosphere with low density.The depth and scale of subduction are controlled by the degree and location of the development of the "plastic layer" in the crust or the lithosphere.Deep subduction leads to reverse detachment and breaking of the middle and upper crust,which then leads to basin orogenic belt occurred in multi-level inverse fault pattern.In this process the "plastic layer" plays the role of dividing the deformation layer,such as fracture,fold,etc.The shallow structure of this type of thrust belt has obvious zoning and subsection characteristics,rock fragments overlapped mainly to shorten,the opposite relationship with the orogenic belt uplift basin subsidence,development of hidden triangle or thrust structure.No "root" type of thrust belt due to the overall rigidity of the lithosphere,deep and shallow in the basement near the surface deformation decoupling.The strong structural deformation is mainly concentrated in the shallow layer,which is characterized by the multi level detachment and breaking of the sedimentary cover.The structural deformation of this type of thrust is characterized by a transitional zone,and the subsection is weak.It has a wide slipping and breaking fold,and the orogenic belt synchronously increases with the basin.The process of thrust and evolution of the large central and western piedmont belt is consistent with the episodic closure and collision period of the new Tethys since Mesozoic.Based on the analysis of the deep and shallow geological structure of the typical piedmont belt,the geometric patterns of 3 types of front edge breakup of the leading edge breakthroughs,the triangulation type and the nappe type are divided.The first type of the foreland belt has the following deformation characteristics.The formation shortening is regulated by the slippage of the "weak layer" in the sedimentary caprock,and passes through the extrusion stress to the leading edge,forming the surface thrust and fault related folds,mainly developing typical imbricate thrust or double thrusting structures.The Dabashan thrust belt,the east Sichuan thrust belt and the Liupanshan thrust belt are typical of this type.The formatting shortening of the second kind of thrust belts is reduced and absorbed by passive reverse thrust faults in the sedimentary caprock,and the thrust toward the orogenic belt is passive.It is typical of the northern part of the Longmen thrust belt and the thrust belt of Micangshan,etc.For third types of thrust belts,In the front part of the rapidly uplifted orogenic belt,the crustal thrusting fault absorbs most of the tectonic variables.The tectonic deformation is concentrated around the main fault.The deformation in front of the main fault is relatively small in terms of shortening or structural displacement.The typical thrust as Bogda thrust belt,Kuluketage thrust belt etc.Based on 3 types of geological models,the corresponding geological model and its structural physical simulation experiment model are established.Based on the aboved model,several single factors and physical simulation experiments have been carried out,such as free detachment layer,base surface / upper shell slippage and double slide layer.Under the experimental conditions of the upper crust and sedimentary strata without slippage layer,the basement involved structural deformation mainly occurs,which is mainly composed of high angle thrust faults,and the structural deformation is mainly concentrated near the boundary fault,and the deformation range is narrow.On the contrary,it is an early manifestation of imbricate thrust structure,advanced the evolution of stacked or stacking triangle belt,deformation zone gradually narrowed as the extrusion continued.The experimental results show that the boundary mechanical conditions and the deformed medium conditions are the main controlling factors for the control of the deformation structure of the sedimentary strata in piedmont belt.Based on the characteristics of the deep geological structure and the results of the structural physical simulation experiments,the dynamic model of the thrust in the Midwest is discussed.It shows that the thrust deformation in the piedmont belt of the central and Western China is essentially caused by the different intracontinental deformation response,due to the difference of material composition and structure,under the impact of remote extrusion pressure.Shallow reverse detachment caused by intracontinental subduction,the multi-level slipping and breaking deformation,are the two kinematic and dynamic deformation modes of intracontinental thrust and deformation in the central and western piedmont belt.Based on the deformation mechanism of the piedmont thrust structure in central and Western China,with the principle of "the main detachment layer depth as the main consideration,and the structural deformation style is taken into consideration”,the paper proposed a new scheme for dividing the tectonic deformation in piedmont belt.The scheme puts forward 4 deformable structural belts,such as thick belt,transition I belt,transition II belt and thin skin belt from the orogenic belt to the basin.Based on that,the possible development of the transition zone can be divided into 4 types: the transitional zone type,the transition type I belt type,the transition type I + II type and the transition type II type.The development of deep and shallow slippage layers and the duration of extrusion jointly control the way of transferring thick belts to thin belts.In the development of the slipping layer without the base surface,the transformation of the thick-skinned belts to the thin-skinned belts is realized mainly through the shell source thrust fault.Base detachment is the main way of transforming thick-skinned belts into thin-skinned belts,and the way of squeezing pressure determines the combination type of transition zone formed by detachment.The thick-skinned belt of the typical piedmont zone in the central and western China is converted to thin-skinned belt mainly through the transition zone.The "root" type of piedmont thrust belt are basically through the transition transition II or I + II with conversion,such as the middle and northern part of Longmenshan thrust belt,both sides of the Tianshan thrust belt.The "no root" type is piedmont thrust belt is mainly through the transition transition I zone or no transition zone with conversion,such as Dabashan thrust belt,Liupanshan thrust belt,Bogda thrust belt etc..The present scale of oil and gas is distributed in a specific deformation zone,such as the transition I zone and the transition zone II zone.The matching analysis of the tectonic evolution and oil & gas accumulation process in the Jinma-Yazihe and the Keshen structure zone,which shows that different deformable zones have their own characteristics of the distribution of hydrocarbon and the key elements of enrichment.The key to the enrichment of oil & gas in thick zone is the spatial matching of the reverse nappe zone and the hydrocarbon generation sag.The accumulation of oil & gas is mainly located in the footwall of the main fault.The reservoir formation period is consistent with the tectonic reversal period,and the oil & gas reservoirs are dominated by lithology,structure and lithology.The key to the accumulation of oil & gas in the transition I zone is the preservation condition in the late period.Accumulation of oil and gas has been adjusted and modified at the same time.Nowadays,most of the oil & gas reservoirs are belonged to residual one,mainly with structural,tectonic and lithologic oil and gas reservoirs.And,the inner front edge of transition I zone is a favorable part of oil & gas exploration.The characteristics of oil & gas accumulation in the thin-skinned zone are similar to those of the transition I zone,and structural development and preservation conditions are the key to oil & gas accumulation.The transition II zone is usually characterized by two stages of oil & gas accumulation.Late stage is the main key period of oil & gas accumulation.The early oil & gas accumulation is similar to the transition I zone.Since the late fragment vertical superimposition,hydrocarbon source rocks generate hydrocarbon again due to being buried,will contribute to oil & gas and secondary hydrocarbon migration and accumulation.The filling condition of late oil & gas is the key factor for accumulation of oil & gas in the transition zone II zone,and the middle and back part of it is the favorable location for oil and gas enrichment.Therefore,the oil and gas exploration in the central and western piedmont belts should focus on the transition I zone and the transition II zone which are the two favorable accumulation zones of oil and gas.On the basis of further research on key factors of oil and gas enrichment,the favorable zones and their exploration targets are further implemented.