Tectonic Evolution And Its Control For Hydrocarbon Accumulation Of Mesozoic-Cenozoic Multicycle Superimposed Reformation Basin In The West Of Northern Qaidam Basin

The basin is one of the basic structural units developed on the continental lithosphericsurface, which evolution is controlled by the earth geodynamic mechanism. Because of thetransformation of the earth's heat and tectonic system, the plate, lies on the lithosphere surface, diverged and converge time after time, and happen multistage intracontinental subduction aftercollision, then, the lithospheric movement has the characteristic of multistage and multicycle. Forthe control of multicycle crustal movement, the basin evolution underwent complicated tectonicsystem transform and showed frequent cyclicity, and the developmental environment, sedimentation mechanism and fill mode frequently changed in the cause of evolution. The basin, which have abundant hydrocarbon resources in the West China, have many original basin phasesin Mesozoic-Cenozoic, but due to multiformity, complexity and delamination of the continentalstructure, the study about tectonized mechanism, evolution and dynamic course in the process oforiginal basin's transform is beyond classical two-dimension plate tectonic theory, it need newtheory to support the research on the bases of three-dimension. Consequently, it not only hasimportant practicability to study the tectonic evolution to the multicycle superimposedreformation basin, and it is helpful to develop continental geodynarnics in the research of thetectonic system transform in the basin evolution.Qaidam basin lies in the northeast margin of Qinghai-Tibet Plateau, and located in theconjoint zone of Asian structural domain and Thetys-Himalayan one in the geotectonic situation, and surrounded by Altyn Mountain, Qilian Mountain and Kunlan Mountain, so its evolution hascomplicated geodynamic mechanism and reformation characteristic in Mesozoic-Cenozoic. Thewest of Northern Qaidam basin is the distributed area of Jurassic petroleum system, and hasabundant hydrocarbon resources. The basin formation processes correlates mainly with AltynMountain and Qilian Mountain in Mesozoic-Cenozoic. Because of multicycle orogenicmovement of periphery orogene, The west of Northern Qaidam basin has multiple phase basinformation and complicated tectonic evolution, and to lead the intricate tectonic reformation, control the distribution and evolvement of source rock, the form of source-reservoir-capcombination, the development and distribution of trap, hydrocarbon accumulation and reformation, and restrict the petroleum multiphase migration, accumulation, reformation andreconstruct, therefore, the distribution of oil and gas in the strata is very complicated. With thegeologic researches had been doing connected with oil exploratory, however, was restrained bythe target of the project, and there are so many cognitions need to unify and so many geologicproblems need to do further research, such as, basin formation dynamic mechanism, tectonicevolution, the distribution regularity of petroleum reservoir etc. In multicycle superimposedreformation basin, tectonic evolution is the ruling factor to control hydrocarbon accumulation, itdirectly restrict hydrocarbon generation, migration, accumulation, reconstruction andpreservation, so the research of hydrocarbon accumulation to the basin must make certain thetectonic evolution above all, and the understand to the distribution regularity of petroleumaccumulation must base on the study to structure. Aim at this main problem in the study ofhydrocarbon accumulation in the west of Northern Qaidam basin, bases on the theory of basinformation and petroleum accumulation, the paper carded the field geologic investigation, theexplanation of seismic profile, analysis of sediment provenance, ESR dating and fission trackthermchronoiogy analysis, analyzing the typical petroleum reservoir forming period and process.Throughout above analysis, this paper acquired some new achievements on basin structure, basinformation mechanism and evolution, the control about tectonic evolution to hydrocarbonaccumulation and the contribution regularity of petroleum reservoir.1,The west of Northern Qaidam basin could be divided three stair structural units:Saishiteng-Lvliangshan uplift, Kunsai depression and Mahal uplift, and according to deformationintensity and kinematics, ten secondary structural units are divided on the basis of stair ones. Thestudy area structure is characterized by segmentation along the strike, zonation in the dipdirection and structural layer vertically. Along the deformation direction of Southern QilianMountain, three structural zones are compartmentalized, from north toward south, thethrust-overthrust structural zone, the fault-fold structural zone and the decollement-detachmentstructural zone. The study are divided two structural segmentations fault from west from east byMaxian. According to the difference of structural deformation style, two tectonic deformationlayers can be sure vertically. On the basis of classify principle of structural style and thecharacteristic of tectonic evolution, there are five structural styles in the study area, such aspiedmont basement thrust, piedmont fault-fold, basement thrust of upper deformation layer, shearstructure and thrust-fold of lower deformation layer etc.2,Apatite fission track dating evidences from 21 samples, which distributed in two crosssections of NE-SW direction and SE-NW direction in the study area, shows that the sampleswhich fission track ages are older than their respective strata ages reflect the provenance ages ofthese apatite, the time of tectonothermal episodes is 52～57 Ma,106～110 Ma,133.3～161.9 Maand 205.9Ma. In the dating, the time of 208.9Ma shows the uplift of Altyn Mountain, other showthe uplift of Southern Qilian Mountain. The samples which fission track ages are younger thantheir respective strata ages shows that the maximum burial temperatures for these samplesexceeded the total annealing temperatures of the fission tracks, and reflect three tectonothermalepisodes which happened in 12Ma,50～63 Ma and 89～118Ma respectively. The dating show that there are four uplift phases which corresponding period is late Jurassic-Cretaceous,Paleocene, Oligocene and Miocene in the west of Northern Qaidam basin, but the peripheralorogen have also a uplift phase in later Triassic. Combined with the characteristic of structuralreformation, it is clear that the reformation mode is only uplift in late Jurassic- Cretaceous, upliftmainly and fault-folding weakly in Paleocene and Miocene, fault-folding consumingly inOligocene and Pliocene-Quaternary which is supported by the isotopes of O and C.3,The field investigation indicated that the filler in joint, fault and fracture has the fillsequence: calcite→, quartz→(gypsum, bitumen) in the west of Northern Qaidam basin, and themineral fill corresponding period is Late Jurassic-Cretaceous, Palaeocene-Oligocene andPliocene which indicates multiple phase tectonic movement. There are the angle disconformitybetween Eogene and Upper Jurassic, nonangular unconformity between Eogene and Cretaceous,and between Cretaceous and Jurassic in the study area, then, the formation relation reflect that thecharacteristic of tectonic evolution is differential uplift in Late Jurassic-Cretaceous. The analysisquartz c-axis fabricshow show Jurassic-Cretaceous is the tectonic violence activity period, andtectonic deformation endurance long-lasting period, Cretaceous-Eogene tectonic activitybecome reducible, and Oligocene violence activity again, Miocene reducible.4,Based on the comprehensive research of palaeocurrent and heavy mineral, and combineapatite fission track dating and basin tectonic deformation analysis, we consider the west ofNorthern Qaidam basin is a Mesozoic-Cenozoic basin which is controlled by the conjunctaction of Altyn Mountain and Qilian Mountain which underwent several multicycle orogeny.Multicyclic orogeny led to multiphase basin formation, consequently, based on multiphaseoriginal basin superimposition in space and later stage tectonic activity to early original basin,the concept of multicycle superimposed reformation basin is proposed, consider that:①becauseof the close of Qinling-Qilian ocean trough and the thrust of Qilian Mountain, the original basinis foreland basin in Indo-Chinese epoch;②Early Yanshan epoch, controlled by vertical balancedadjust of crust, the fault depression and depression form in local extendible environment;③Dueto the new Tethyan's open in Late Yanshan epoch, the feeble compressional environment form,the original basin is depression;④Early Himalayan epoch, owing to the collision between Indiablock and Eurasia block, Altyn Mountain and Southern Qilian Mountain revive, Joint actionleads to strike slip-foreland basin formation.⑤Tibet highland epoch uplift and expand again,strike slip-foreland basin further forms in Late Himalayan.5,According to Mesozoic-Cenozoic basin-mountain system in the west of Northern Qaidambasin, the tectonic evolution could be divided into three cycle, four subcycle. Indosinian cycle ischaracterized by thrust-overthrusting and uplifting-erosion. Early Yanshanian subcycle representextension tectonics and develops miniature fault depression. Late Yanshanian subcycle ischaracterized by differential uplifting attend by mildly thrusting. Early Himalayan subcyclemainly take place thrust-overthrusting, fault-folding and strike-sliding. The tectonic evolutioncharacter of Late Himalayan subcycle is the same as Early Himalayan, but become stronger.6,In multicycle superimposed reformation basin, hydrocarbon accumulation responds totectonic evolution, the latter play the role of building and reconstructing for reservoirs. According to the analysis of tectonic evolution and hydrocarbon accumulation in the west of NorthernQaidam basin, The author consider that the tectonic evolution effect constructively on reservoirformation in Yanshan epoch and early Himalayan epoch, and acting as construct-reconstructingin late Himalayan epoch. The constructing of Yanshan epoch behaves as promoting the formationof hydrocarbon-generation sag, coming into being paleo-uplift which is propitious tohydrocarbon accumulating, and making for unconformity transporting pathway for petroleumlateral migration. The constructing of Himalayan represent as accelerating hydrothermalevolution of source rock and generation-expulsion of hydrocarbon, forming structural trap andfault transporting pathway. Construction and reconstruction take place synchronously in LateHimalayan. The reconstructing is characteristic of reservoir uplifting and faulting. As to Jurassicand Paleogene hydrocarbon accumulation, it shows that multi-stage formation and late-stageadjustment for hydrocarbon accumulation. The constructing act on Neogene hydrocarbonaccumulation figure as promoting the formation of structural trap and fault transportingpathway, and characterize by fault transporting and late-stage accumulation of hydrocarbon.7,Based on the evolution of tectonic and analysis of typical reservoirs, consider that thereservoirs are controlled by distribution of effective hydrocarbon source rock and effectivesystems of transmission in the west of Northren Qaidam basin. It is controlled by verticalbalanced adjust of crust, there are several hydrocarbon generating depressions in the region ofstudy, then, the trap may apt to capture hydrocarbon in margin of hydrocarbon generatingdepression, and the scale of hydrocarbon source rock controls the scale of surrounding reservoirs,at the same time, the maturity of hydrocarbon source rock control the distribution of phase ofhydrocarbon. In study area, there is always definite distance between reservoir and hydrocarbonsource region, so the petroleum blow off from hydrocarbon source rock, migrate through somesystems of transmission and accumulate in traps to form reservoirs. The net systems oftransmission which compose of faults, unconformity and transmission layer of sandstone play asignificant role to vertical and lateral migration of petroleum. The transmission capability ofsystems of transmission restrict the migration, accumulation, and forming reservoirs directly.8,In multicycle superimposed reformation basin, the tectonic evolution controls thedistribution of hydrocarbon, and the controlling is embodied as restraint to the distribution ofhydrocarbon generating depression, the thermal evolution of source rock, the scale of effectivesealed systems, the formation of transmission systems, and the development of structural traps.Combined tectonic evolution, consider that the distribution of hydrocarbon has three regularpattern in the west of Northern Qaidam basin: the distribution of source rock and the hydrocarbongenerating evolution control the distribution and phase of reservoirs, palaeohigh of later Yanshanperiod and the structure which developed in Himalayan period are the region to which deeppetroleum apt to migrate, the effective systems of transmission which compose of deep andshallow faults control the formation of reservoirs in Neogene.