Lithosphere Structure Of The Qiangtang Basin, Qinghai-Tibetan Plateau And Its Influence On Petroleum Prospect

Located in the northern Qinghai-Tibetan Plateau and the eastern tectonic realm of Tethys, where is the accumulation zones of the huge oil and gas in the world, Qiangtang basin is a region of the most hydrocarbon potential, and the most intact marine strata in the northern Qinghai-Tibetan plateau. Geophysical data indicate that the Qiangtang basin may be the collision zone between the Indian plate and the Asian plate, and the north-south compressional stress affects the tectonic deformation pattern of the basin. The previous method of oil and gas exploration has used little deep crustal structure information. In fact, deep structure of the basin affects not only evolution of the basin, but also the formation and migration of the oil and gas. Therefore, analysing the lithosphere structure of the Qiangtang basin and searching the effect of the lithosphere structure on oil and gas resources, are the forefront of earth science research, and also the urgent task of perspective evaluation of oil and gas resource.The author completes the paper on the basis of participates in seismic data acquisition and data processing in the project《The investigation and evaluation of strategic precinct of oil and gas resources in the Tibetan Plateau》.(1)The main contents:This paper can be divided into 4 parts by contents. The first part: Researches on the lithosphere structure in the Qiangtang basin. After the data analysis to the previous deep geophysical data, the author summarized the following characteristics of the lithosphere structure in the Qiangtang basin:The Qiangtang basin is located in the front of collision between the Indian plate and Eurasian plate. The subducted Indian plate met with the mantle of Eurasian plate beneath the Qiangtang basin, and entered in the mantle after face-to-face collision. The crust and lithosphere become thin obviously, and the Moho offset occurs in different scales under two sutures on southern and northern boundary of basin, respectively. Lhasa Block to the north basin and the Hoh Xil Block to the south of basin are squeezing into the Qiangtang basin; the large scale low velocity bodies occur beneath the basin has been interpreted to be originated from the going-up of temperature or upwelling of the mantle. There may be channels for the flowing of hot material in partial structural weakness of the crust and mantle, where the large material exchange; after the lithospheric mantle breaking off, the rheosphere mantle upwelling, and the crust was extended near EW direction, and the north-south extensional tectonics occurred in the basin.The second part: Study on the basement and covers of the Qiangtang basin. Because of the complex geology conditions and tough work environment, the oil and gas exploration in the Qiangtang basin has not achieved the breakthrough so far. How the basement and covers of the Qiangtang basin is still unclear. On the basis of the completion of seismic data acquisition and processing tests, a 2D seismic data acquisition and processing techniques in the Qiangtang basin has been obtained:1) Data Acquisition: Exploding in the single well and more than 15 m well-depth must be adopted, the combination of wells is avoided to be used in exploding. The optional explosive is about 16-18kg; The explosive and well depth are guaranteed as the premise, the spread length of more than 7000 m and record length of more than 10s can ensure the effective reflection of basement; Reasonable big explosive (100 kg ) can increase downward energy and improve resolution, and can obtain effectively the basement reflection. At the mean time, the reflection energy of a relatively stable layer can be enhanced to provide standard stratum for residual static correction in the processing. To track changes of the structure of the same area in different directions and interpret the structure stratum of the area, the cross section is needed to emplace.2) Equipment: Drilling is the most important. The high power and vehicle-mounted rig must be equipped. Two MN rigs with 300 horsepower, made in German used in 2004 and a rig with 260 horsepower was used in 2006.They can work in a variety of geological conditions. Where vehicle can't arrive, the portable diesel rigs were available. The advanced acquisition equipment is a guarantee in acquiring a high quality seismic data. The 408 seismograph has many advantages to ensure the good effect in the Qiangtang area, such as light cables, multiple receivers, acquisitions of higher resolution;3) Field Monitoring:Outfitting the field monitoring equipment with high-performance. Processing raw data and provide monitoring profiles in time are helpful to guiding next work, which is a necessary measure to ensure quality.4) Data processing: The seismic data needs fine processing and special processing. Fine static correction procedure must focus on the effective static correction method. Comprehensive application technique of the three surface static correction method and non-ray tomography static correction are fairly effective on the processing of seismic data from the Qiangtang basin; Carefully analyzing signals and noises is helpful to avoid the loss of useful signals during removing noises. The tests showed that the adaptive noise suppression and the various noise removal means used for different noises can greatly improve the fine resolution of the profile; Explaining the velocity spectrum, combining with constant-velocity scanning, scanning velocity and the layer velocity analysis can identify effectively the velocity changes. Though creating more accurate velocity model and using an automated residual static correction, the precision of conventional velocity is enhanced.Through processing of the new and the old seismic data, we obtained the profile for the structure explanation. Some new understandings to the tectonic framework of the Qiangtang basin are obtained after analyzing and explaining the seismic profiles:The reflection seismic profile from the Qiangtang basin may divide into three sections in ordinate direction, the upper Mesozoic strata from 0 to 2.0 s (TWT) appears the intensive fold deformation. The reflections from 4.5s to 6s may represent the basement of basin. The flat Paleozoic stratum lies on the basement and Proterozoic stratum is under the basement; the central uplift divides the basin into the south Qiangtang basin and the north Qiangtang basin. There are 9 faults in the basin distributing in NWW or NW direction, which control the tectonic framework of the Qiangtang Basin; the central uplift extends eastward at least to 89°42 '. The span of the central uplift from west to east gradually decreases from 61km to 40km.The basement of central uplift is shallower and become deeper gradually to both sides. The deformation of the basement control the basal outlook of the shallow stratum in the basin, the shallow arcuate dome and the basement uplift have the continuity to some extent.The third part: How the deep structure influence on the basement and covers is discussed in the part. In the northern Bangong - Nujiang suture zone, the basement of the basin is affected obviously by northward movement of lithosphere of the Indian plate, the upper basement appears a discontinuous arc-shaped reflection caused by the subduction; The tectonic deformation in the northern Qiangtang basin is weaker than that in the southern Qiangtang basin according to the seismic reflection profiles. The Qiangtang basin suffered from the tectonic compression in the north-south direction, resulting in the structural deformation with various degrees and forms. The deformation of the north Qiangtang basin is weak because of the obstruction or absorption of the central uplift, and thus benefit to the preservation of oil and gas.The fourth part: The author gives a preliminary discussion on the oil and gas prospects in the Qiangtang basin on the basis of the above work:The obvious thermal feature occurs in the deep crust, indicated by the thermal activity and the magmatic distribution in the shallow level in the Qiangtang basin. This is the necessary temperature conditions for the formation and evolution of oil and gas fields. The deformation of Mesozoic strata is intensive, the Paleozoic strata with the relatively weak deformation can be regarded as another important layer for oil and gas exploration; The half-graben structures developed in the Qiangtang basin have the strategic value for oil and gas survey; Contrasting the south Qiangtang and the north Qiangtang basin, oil and gas prospects in the north Qiangtang is better than that in the south Qiangtang. There is a possibility that oil and gas may exist in the deep Qiangtang basin. Because of the collision of the Indian plate with Eurasian plate, the lithosphere of the Qiangtang basin has developed the intensive deformations, and has formed a series of fault-fracture system. They provide a good channel for migration of non-biogenic gas from the deep in the high temperature and pressure conditions to the upper low temperature and pressure conditions.(2) Main results A feasible program valid in prophase of the seismic exploration was brought forward in the article, by detail gathering tests on the 2D seismic reflection data completed in Qiantang basin. Using new and reprocessed seismic data, the author made out a long seismic profile across the Qiangtang basin body, which is the first time in the seismic exploration history of this area. On the basis of the above works, the author gives some main results:1) Lithosphere structure of Qitangtang basin: North-South compression,Moho break-off under sutures,thinned crust and obvious thermal action are main lithospheric characters of Qiangtang basin.2)Structure of basement and covers of Qiangtang basin: It behaves three parts in depth on the seismic reflection profiles. The upper Mesozoic strata from 0 to 2.0 s (TWT) appear the intensive fold deformation. The reflections from 4.5 s to 6 s may represent the basement of basin. The flat Paleozoic stratum lies on the basement and Proterozoic stratum is under the basement. 9 faults in the basin distributing in NWW or NW direction control the tectonic framework of the Qiangtang basin.3) Study on the 2-D reflection seismic acquisition method in the Qiangtang basin: Exploding in the single well with more than 15 m well-depth, 16-18kg optional explosive; more than 7000 m spread length and more than 10s record length; reasonable big explosive (100 kg) and the cross section are effective acquisition technique for prophase exploration in Qiangtang basin.4) Oil and gas prospects for the Qiangtang basin: The obvious thermal actions occurring in the deep crust afford the necessary temperature conditions for the formation and evolution of oil and gas fields; With the relatively weak deformation, the Paleozoic strata can be regarded as another important layer for oil and gas exploration; The half-graben structures developed in the Qiangtang basin have the strategic value for oil and gas survey; Contrasting the south Qiangtang and the north Qiangtang basin, oil and gas prospects in the north Qiangtang are better than that in the south Qiangtang for weak deformation.