The Tectonic Evolution And Lower Paleozoic Fracture Characteristics Of Organic-rich Shale In Qianxibei Area

The Qianxibei area is located on the Yangtze platform and its tectonic unitincludes: the central guizhou uplifth, the southeastern part of Dianqiannortherndepression and the southeastern small part of southern Sichuan depression. TheQianxibei area is a part of the Yangtze platform. In a sense, its tectonic evolution obeythe regional tectonic evolution of the Yangtze platform. Certainly, the tectonicevolution of the Qianxibei area has its own characteristics. The all Yangtze platformexperiences seven period large of tectonic: the Xuefeng Movement period, theearly-middle Caledonian Movement period, late Caledonian Movement period,Hercynian period, Indo-China Movement period, Yanshan Movement period andHimalayan Movement period.The folds and faults of Qianxibei area are very developed. The structuraldeformation is in the main of extrusion, and it also has strike-slip nature. The most ofthe whole fold strikes are NE and NNE, and the fewer are NW. Most folds which havethe characteristic that the anticlines are wide and box-shaped and the synclines arenarrow and trough-like are trough-like folds. The tectonic denudation results in thelandscape with "a mountain syncline, a valley anticline". There are four sets of faultsystems which include north-east, north-north-east, north-west, east-west in Qianxibeiarea. The main strikes of the faults are north-east and north-north-east, andtheir inclinations are large.The main tectonic unit is the central guizhou uplifth, so the tectonic evolution ofQianxibei area is embodied in the formation, development, prosperity, decline andextinction of the central guizhou uplift. The central guizhou uplifth experiences theperiod of Yunan Movement of underwater micro uplift stage, the period of DuyunMovement of succeeding-development and prosperity stage and the period of ZiyunMovement and Dongwu Movement of decline and extinction stage.Tectonic fractures of the core is very developed in four shale gas survey wells inQianxibei area. The length of the fractures and their inclination are positivecorrelation. The fractures are closely linked with tectonic. Generally, the fractureswhich have long length, large opening and high inclination are developed where the folds and faults are developed. The fracture fillings is brittle mineral which will helpthe development of shale gas.The high value of tectonic stress field is concentrated in the union interchange offaults, faults convex areas, ends and conversion and the high steep site of folds.Tectonic stress is the main cause to fractures, so it is very easy to generate thefractures in these tectonic stress concentration areas. And the fractures are verydeveloped in these areas. The predict developed fractures of Niutitang organic-richshale on the Lower Cambrian are mainly concentrated in developed areas of faults insouthern area, and developed areas of faults and the high and steep parts of folds innorthern and north-eastern some areas. The shale of Lower Silurian which only hassedimentary in parts of northern area and north-eastern area is corroded severely. Sothere are developed fractures only concentrated in parts of faults and the high andsteep parts of folds of these regions.