The Characteristics Of Oligocene Tectonic Fractures In Central Qiangtang Basin, Tibet, China

Qiangtang Basin is located in the hinterland of the Qinghai-Tibet Plateau, which permafrost distributed.In recent years, studies of natural gas hydrate in Plateau Permafrost have been more and more popular.The study area hydrocarbon migration times, processes and tectonic setting is very important..To answer that, the paper explores the developmental characteristics of the fracture in Qiangtang Basin in middle and its tectonic significance of gas hydrate accumulation.,by means of drilling core log, fracture statistics and ESR dating, etc.This paper conducted a detailed catalog with the core and take a statistical study on the characteristics of fracture. The drilling stratum of QK01 are the Xiali group (J2X) and Buqu group (J2b), whose lithology are respectively carbonate rocks and dark gray shale,and the shear fracture which is nearly 70°evelop in the stratum.The drilling strata of QK02 is the Xiali group (J2X), whose lithology are fine clastic rocks and carbonate rocks, and intense dissolution seam occurs within the structure fracture of core in the latter, and there are traces of heavy hydrocarbon migration in the fracture. The drilling stratum of QK03 are Quse (J1q) group and Kangtuo group (E2k), whose lithology are dark gray mudstone,purple mudstone with sand-based, and shear fractures which is intersect rock wich a high angle develop in the stratum and cracks scratches is seed in the surface of fractures. The drilling stratum QK04 are Kangtuo group (E2k) and Suona Lake group (E2S), whose lithology are mainly mudstone with fine sand interbedded with siltstone,and core fractures are filled with gypsum in multi-stage. The drilling strata of QK05 is Bailongbinghe Group (J3b), whose lithology is marl, and structural fractures are filled with calcite and bitumen developing in strata. According to the shape, open degree and filling etc. of core fractures this paper identify four types fracture,such as structural fractures, dissolution seams, seam structure dissolution, pressure solution seam. Crack is overall relative vertical, and two types line management is see in the surface of some cracks, which is mostly slip lineation. That the fractures are mainly structural fractures and structural solution seam indicated the main controlling factor of cracks is tectonic. Based on the literature study and cracks in the process of development, these cracks will be characterized as the initial cracks.To explore the significance of regional tectonic fracture drilling, the paper area and drilling crack crack a comparative analysis. Field surveys show that the region in general developmental Cenozoic strike NW or NE conjugate shear joints, the initial rupture characteristics. In Oligocene Kangtuo strata within the group folds nearly EW relief anticline is characterized by the development of small-scale reverse faults, shows a weak extrusion characteristics, which is drilling the initial rupture characteristics can be matched. Stereographic projection shows a nearly north-south extrusion characteristics, thus we believe that nearly north-south squeezing action of tectonic fractures may control the formation and distribution of the region. Cracks in the region predecessors calcite ESR dating also shows cracks formed mainly in the area since the Oligocene, which recognize and paper match.We find the core fractures is filled with calcite which is five stages at least. A ESR dating results for one newer calcite that this paper provides is 1.1±0.2Ma. and the Plateau had been frozen stage which is showed by datas.At a result, if the event had been hydrocarbon expulsion.it is a important significance for permafrost gas hydrate exploration and research to study deeply these tectonic fractures。...