Gas-bearing Ability Evaluation Of Shale In Northwest Guizhou

As one of the China’s marine shale gas exploration potential favorable zones, The Northwest Guizhou which is located in the southwest of Upper Yangtze plate have excellent shale gas accumulation conditions. The paper studied geological background of organic-rich shale reservoir-shale gas formation conditions-characterist ics of containing shale gas-comprehensive evaluation of shale gas main controlling factors, analysed the the ability of shale gas contain in Paleozoic of the Northwest Guizhou, and have important theoretical and practical value.Northwest Guizhou experienced multiple stages of tectonic reforms. There are the attributes of this region such as adjacency of upheavals and depressions, the distinction between the north and the south, the combination of wide spaced anticlines, the integration of "S" type structures and anti "S" type structures. In research zone, the sedimentary fancies are deep water shelf for Nuititang formation, shallow to deep water shelf for Wufeng and Longmenxi formation and offshore lacustrine basin to marsh for Longtan formation. There is the depocenter in the range from Qianxi to Xiuwen for the shale of Niutitang formation. The depocenter for the shale of Wufeng and Longmaxi formation is to the north of Jinsha and Zunyi, which presents the characteristic as becoming thicker to the direction of Chongqing. The depocenter for the shale of Longtan formation is in the range of Qianxi and Dafang.The types of organic matter contained in two series of lower palaeozoic shale are mainly type I including the trace of type Ⅱ1 However, type Ⅱ1 is rather dominative of upper palaeozoic shale. TOC content is about 0.50%-6.96% for Niutitang formation,0.31%-3.34% for Wufeng and Longmanxi formation and 0.54%-17.03% for Longtan formation, respectively. The evolving degree of shale is quite high for all formations. It is about 1.55%-5.50% for Niutitang formation,1.42%-3.21% for Wufeng and Longmanxi formation and 1.57%-2.66% for Longtan formation, respectively and generally, which presents the feature of over maturity. The fragile mineral content of most black shale were more than 40%, the main fragile mineral is quartz, clay mineral content is less than 50%, among them goeschwitzite and andreattite are the main mineral composition. The pore types of black shale is rich, the most are micron- nanometer pore. The permeability of most black shale are less than 0.01mD, porosity mainly distribute between 3% and 8%, porosity and permeability have positive correlation, suggesting that the two marine shale has good reservoir conditions. The result of well site desorption, logging evaluation and isothermal adsorption simulate test showed that, the gas content of Niu-Titang formation, Wu-feng formation and Long-ma-xi formation of black shale mainly distribute between 0.5m3/t and 20m3/t.The controlling factors of shale gas accumulation is the shale own geochemical indices, mineral composition, physical indicators, etc. The external controlling factors include depth, temperature and pressure, etc. Among them the total organic carbon content, clay mineral content, quartz content, BET surface area, BJH total pore volume, pyrite content are the main controlling factors of shale gas accumulation.With the depths of the shale formations decreasing, gas content increases correspondingly. However, for the single formation, there is a positive relationship between gas content and formation depth. For the shale of all formations, it can be divided as high quality, secondarily high quality and general gas bearing shale. Compared the main gas bearing shale of U.S. against China, the source shale in research zone can be attributed to the characteristics as the following:multiple potential gas bearing shale, favorable thickness, high TOC content, high maturity, high porosity, high permeability, high gas content, remarkable potential of shale gas resource and favorable exploitive value for gas industry.