Qaidam Basin In West Himalayan Movement And Hydrocarbon Accumulation

The author analyzed tectonics of Qaidam basin and plate evolution process, further make definite that Altin fault zone mainly activated during Mesozoic and Cenozoic. The author discussed the mechanism of Cenozoic Qaidam basin development and controlling factors. The Qaidam basin formed with the control of boundary activation and deep-seated structures, and the migrations of depocenter and subsidence center are related with the activation of deep-seated structures. The earthquake data, geophysics of deep-crustal structure, sediments and geothermal data show that the depocenter of Quaternary is controlled by deep geology; paleogeothermal data of west of Qaidam basin,faults structures and sediment characters implied that this area underwent mantle uplift during the early Himalaya Orogeny because of extension of supracnist developed following Altin and east Kunlun faults sinisterly strike slip. The tectonics dynamics which control migration process of depocenter during Cenozoic is the lateral shortening and strike extension of deep-seated structure. Basin basement and supracnist activate with the middle crust as plastic strata which result the crust under depocenter become thinner and subsidence enhanced and form the depression regions.The author discussed with detail the structure deformation, deep geology character of west of Qaidam basin, summarized dynamic features of Himalaya stages of west of Qaidam Basin from integration of geology, 2D/3D seismic data, remote sensing, gravity, aeromagnetic and well data. During the later stage of Himalayan orogeny, Aktenen mountain(part of Altin Zone, west of Yingxiongling Uplift) rotated clockwise and uplifted caused by SN regional compression, while Ganchaigou zone(middle part of west YL Uplift) developed a SE local compression stress field, which resulted the basement uplifted rapidly. In the near southern area of YL, local extension tectonic stress field developed, and a Neogene-Quaternary lake formed. The middle stage of Himalaya orogeny transformed the paleogene transtensional tectonic setting to down-warping in West Qaidam Basin. Altin mountain rise, sediment center migrated to east and north. By analyzing the major faults, tectonic blocks and sediment depression distribution, the author realized that YL and adjacent area developed local extension tectonic setting, formed the stable fault-controlled basin, fine source rocks developed.Geochemical data shows source rocks distribute in several segmented depressions which generated different hydrocarbons in mother material types, redox conditions and salinity. The major reason is that tectonics controlled the depth of depressions, basement activation and supplement of sediments. Faces, sediment systems and sequences studies imply that depressions are controlled by tectonics.During the mid Himalaya orogeny, Altin mountain in south west of Yingxiongling began to uplift and unconformity between Pliocene and Miocene can be observed in Qigequan section. This regional uplift generated the first stage structural fracture and resulted the down-warped basin developed from fault depression setting. The down-warped process accelerated source rocks maturation. With good match of fracture developing and maturation of source rocks, the first stage of generation and migration of hydrocarbon appeared. Synsedimentary structures are favorite traps of this stage.The late Himalaya orogeny was very strong in west of Qaidam basin, which resulted severe deformation, displaying as strong compression-shear movement. Neotectonics since Quaternary is displayed as apparent uplift in west of Qaidam basin, but with different boundary conditions, different sectors have their own structure activations. For example, Gaskule area developed a Quaternary down-warped depression while Yingxiongling uplifted apparently, and local structurezones and depressions controlled by compression-shear activation of faults developed in northern of Yingxiongling. The late Himalaya orogeny controlled" late stage accumulation of hydrocarbon, and structure traps developed in this stage are favorable for petroleum seeping from destroyed paleo-reservoir.The late Himalaya orogeny produced large number of fractures. The regional SN direction compression make SN trend fractures be the effective discharge system, and the generated NW and NE trends shearing fractures are also effective during tectonic activation. So the oil fields in west of Qaidam basin have the character of late accumulation. With late Himalaya orogeny destroying primary reservoirs, secondary reservoirs appear in structures or non-structure traps developed during late Himalaya orogeny stage. The perspective exploration targets include fracturing traps, stratigraphic traps and Oligocene epoch raised fault blocks.