Study On Structural Characteristics And Tectonic Evolution In The Northern Slope Of Central Sichuan Paleo-uplift And Their Influences On Hydrocarbon Accumulation

The study area is located in the northwest wing of the eastern section of the axis of Central Sichuan paleo-uplift in Sichuan Basin.Because of the present structural morphology tilting to the northwest with a monoclinal structural belt,so the study area is called as the northern slope of Central Sichuan paleo-uplift.The west side of study area closes to the primary hydrocarbon generation center,showing an extremely superior source and reservoir matching condition.It is one of the key blocks that have made great breakthroughs in the Sinian-Lower Paleozoic oil and gas exploration in Sichuan Basin.However,there are still some problems of the relevant researches on structural characteristics and tectonic evolution.Firstly,the spatial distribution of unconformities,the direction and scale of strata denudation in the study area need to be refined.Secondly,the geometry,kinematics and dynamics of fault systems require to be studied urgently.In addition,how the paleo-tectonic patterns of the study area were distributed in different periods of geological history demand to be detailed analyzed.In order to comprehensively grasp the structural characteristics and evolution of the northern slope area of the paleo-uplift and deepen the understanding of influences of structure on hydrocarbon accumulation,the structural characteristics and tectonic evolution are analyzed in detail,which is based on the theory of basin dynamics,used to the latest regional geology,drilling data,well logging data and the latest 2D and 3D seismic profile data.Lastly,combining with the elements of oil and gas accumulation,this paper reveals out the structural characteristics and evolution of the northern slope area and its influence on oil and gas accumulation,so as to lay a solid theoretical foundation for expanding the new field of oil and gas exploration.The main achievements obtained through the research are as follows:(1)Based on outcrop,drilling and regional seismic datas,the types,spatial distribution and denudation characteristics of the the unconformity formed during the key tectonic transformation period in the study area are determined in detail.The study shows that there are three periods of regional parallel unconformity in the northern slope area of the paleo-uplift:Tongwan period,Caledonian period and Indosinian period.Firstly,the unconformity of Tongwan stage is characterized by regional Parallel unconformity contact between the Z₂dn²and Z₂dn³of the Sinian Formation,the Sinian system and the bottom of Lower Cambrian.The unconformity shows a serrated stripe running through the north and south sides of the study area,and the denudation zone of the pinch-out line between Z₂dn³and Z₂dn⁴of the Sinian Formation covers an area of about 12,000 km².Secondly,the Caledonian unconformity is characterized by the extension-angle unconformity contact between the bottom of Permian and each set of Lower Paleozoic strata.From SW to NE direction,the eroded strata are changed from old to new,and the denudation spread looks like as a nose-like structure plunging towards NEE.Thirdly,the Indosinian unconformity is distributed throughout the northern slope area,and the top of T₂l⁴Formaiton are missing regionally,showing that the Xujiahe Formation of the Upper Triassic is parallel to the Leikoupo Formation of the Middle Triassic.(2)Through discussing and analyzing the application conditions of methods for recovering denudation,combining with the tectonic-sedimentary evolution history of Sichuan Basin,the denudation amount of Tongwan unconformity and Caledonian unconformity are rescovered by using the impression method and the geological comprehensive trend analysis method.The main denudation area of Tongwan unconformity was concentrated in the Deyang-Anyue rift,and the maximum denudation thickness was 260 m in the Lezhi area.The denudation thickness of the strata in the west side of the study area was less than 100 m,and in the east side was ranged from 130m to 150 m from Pengxi to Yuechi area.The denudation thickness of Caledonian unconformity gradually decreased from the paleo-uplift axis which was ranged from 750 m to 900 m to the northern side,in which only about 150 m of denudation in Jiange to Langzhong and Yingshan area.(3)Based on the latest 2D and 3D seismic fine structural interpretation and seismic attribute analysis,it is pointed out that the faults of the middle-deep systems passed down through the Sinian system and up to Jialingjiang Formation of the lower Triassic in the profile.Taking Jialingjiang Formation as the boundary,the structural styles of faults in shallow and middle-deep systems are quite different.The faults in shallow are characterized by thrust faults formed by compression,while the faults in the middle-deep strata are characterized by tensile normal faults,the single high steep vertical faults,"flowers"and"Y"structural styles.In the plane,the main faults mostly extend linearly,while the small faults are arranged along the main fault zone in oblique row,mainly distributed in the south of the north slope area,with the characteristics of more south and less north.The fault strike is multidirectional,mainly NEE,NWW and near NE.Combined with the plane and section characteristics,it is considered that the boundary tensile normal fault and the tensile and torsional strike-slip fault are developed in the North Slope of paleo-uplift.(4)According to the age analysis method of fault cutting strata,fault-distance analysis method,and balanced profile evolution technology,the activitives of faults are clarified.It is shown that the activity strength of different strike-slip faults in the study area is different.The same fault zone shows obvious zoning and segmental deformation in the plane,and has significant stage-stratified differential activities in the longitudinal direction.Among them,the segmental deformation of F3 fault,F6fault and F7 fault were particularly obvious,and the stratified differential activity of F1 fault and F2 fault were most significant.On the whole,the faults show multiple stages of inherited development,which had experienced the period of Tongwan,Caledonian,Hercynian,the Early Indosinian and Since the late Indosinian period.And the Tongwan extensional normal faults,the Caledonian to early Indosinian tensional and torsional faults and compressional thrust faults since late Indosinian faults were developed respectively.(5)From analyzing the dynamic influence factors of tectonic activity,it is considered that three main factors,including the regional stress field,the weak structural zone of the basement and the zone detachment layers,control the characteristics of zonal,segmental and stratified deformation of the faults in the north slope area.And the regional tensional action is the main reason that causes the strike-slip faults to slip in the study area.(6)Based on the comprehensive analysis of the matching relationship between structural characteristics,evolution and hydrocarbon accumulation factors,it is considered that the structural characteristics and evolution of the north slope area of the paleo-uplift affect the spatial allocation of hydrocarbon accumulation factors such as source rocks,reservoirs and traps.The north slope area was located in the relatively high part of palaeo-uplift at the peak of oil generation before Permian,and had became a favorable direction area for oil migration.It evolved into the upper slope zone of the Central paleo-uplift in the oil and gas cracking period,which promoted the oil cracking in the late Triassic.The extensional faults had controlled the sag-platform sedimentary pattern of the Sinian Dengying Formation in the northern slope area.Furthermore,strike-slip faults in the direction of NE,NWW and NEE might controlled the micro-geomorphic pattern inside the platform and intensify the differential distribution of grain beach facies.The surface karstification of Tongwan and Caledonian unconformities controlled the formation and distribution of large-area carbonate-karst reservoirs.On the whole,strike-slip faults in the key oil generation period were active continuously,effectively communicating with the source and reservoir,while the strike-slip faults in the key cracking period had less activity,which was conducive to reservoir preservation.