Remote Sensing Finding of Vortex Structure Deformation in the Northwest of Qinghai-Tibet Plateau and Its Significance of Geodynamics and Oil and Gas Geology

Key words: 
• Qinghai-Tibet plateau /
• remote sensing /
• vortex structure /
• geodymmics /
• oil and gas

Abstract: By remote sensing technique, we found four vortex structures,including the vortex structure of the west Qilian mountain, Saishenteng mountain vortex structure of the north margin of Qaidam,Qimantage mountain arcuate shear structure of the southwest of Qaidam and big scale vortex structure of the east of Qilian mountain. We analyzed geodynamic mechanism of these vortex structure formation. As for the genesis of the vortex structure, many people have discussed it before. Li Siguang studied its forming mechanism from the change of earth spinning velocity. We believe that it was the result of the long range effect of structure active stress of plate margin affected intraplate. From the early research achievements in the region and map of China satellite image (1∶2,500,000), forming present tectonic framework of the northeast of Qinghai Tibet plateau stems from three forces. Thefirst is that Indian plate underthrusts to the north, and collides Eurasian plate, and pushes the south of Eurasian plate by big M shape since the late Cretaceous; the second is that obstruction of Siberian plate intensely shifting to the south, and relative Monglian arc pushing Indian plate since Cenozoic; the third is that obstruction of Erduosi block when Qinghai Tibet plateau pushing to the east. The first is the main among three forces. The three together caused the northeast of Qinghai Tibet plateau into the large compresso stress field. By further research, late Cretaceous Tertiary principal stress is about SN, which caused a series of NW right lateral strike slip faults in the north of Qinghai Tibet plateau. When mass between two NW right lateral strike slip faults moved to the west, it existed left lateral tendency; when encountering obstruction of Tarim block, it further rotated to the left,that is the reason that forming a series of vortex structures of cyclic plane rotating to the right, such as the vortex structure of the west Qilian mountain, Saishenteng mountain vortex structure of the north margin of Qaidam and Qimantage mountain arcuate shear structure of the southwest of Qaidam. Since Pliocene, main stress turned to NNE NE. Affected by change of regional stress field, Arjin fault was intensely left lateral and lead to the north of the whole Qinghai Tibet plateau escaping to the east, whose results were that forming rotation of cyclic plane anti clockwise, when Qinghai Tibet plateau escaping to the east and Erduosi block obstructing it. Present intensely active big scale vortex structure is the evidence of the kinetic process in the east of Qilian. We also analyzed the relation of these vortex structure with oil and gas distribution. Generally speaking, twisted anticline belts emerge at the end of the twisted vortex structures, whose types of oil and gas reservoir are main anticline reservoir. In the head of vortex structure, there exists the most intense shear stress, which appears intense tenso shear stress area in the outer of head and intense compresso shear stress area in the inner. The former leads to tenso shear stress fault and relative tension crack which formed fault block or structural fracture reservoirs, and the latter leads to compresso shear fault block, which formed complicated fault block reservoirs. For the condition of oil and gas migration and congregation, the positions of stress concentration or strength, such as the nuclear column of vortex structure or convergent position of broom shape structure, are advantageous for forming deep oil generating whirl structures, and disadvantageous for congregation of oil and gas. But the positions of the end of vortex structures or scattering part of broom shape structure, are advantageous for forming diagonal shear anticline and the pointing position of oil and gas migration and congregation. This paper pointed out that the theories that Tapponnier put forward "strike slip line field" and structure escaping" had two distinct insufficience and at the same time, our findings and studies are supplememt to these theories.We also pointed out