Geological Conditions Of Shale Gas Formation And Favorable Area Selection In The Upper Yangtze Region

The great success of U.S. shale gas exploitation industry，which provoke people’s great interest of shale gas exploration prospects in the Lower Paleozoic of the Upper Yangtze region, China. Therefore, it is necessary to change in exploration strategy, using shale gas theory to guide oil and gas exploration, in order to achieve a breakthrough of gas in organic-rich shale. The paper studied geological background of organic-rich shale, reservoir-forming conditions and main controlling factors of shale gas, strategic select potential areas of shale gas reservoir development, Geological analogy method used on the analysis of reservoir-forming condition of shale gas in the Lower Paleozoic of the Upper Yangtze region, and used comprehensive information method to predict initially the favorable areas for shale gas reservoirs.The Upper Yangtze region had undergone multiple phase reform in tectonic evolution, has the characteristics of early clarification, late uplift clarification of the long and short settlement; Niu-Titang formation of lower Cambrian system black shale develop in the deep water environment, which were the deep water continental shelf and deep water continental shelf of hot water zone. Wu-feng formation of upper Ordovician system and Long-ma-xi formation of lower Silurian system black shale was controlled by the deep water continental shelf facies, which develop also in the deep water continental shelf zone; The center of Niu-Titang formation of black shale were Southern Sichuan Yibin and Western Hunan-Eastern Chongqing and Western Hubei, the center of Wu-feng formation and Long-ma-xi formation of black shale were Southern Sichuan Yibin-Changning-Luzhou, Eastern Chongqing Shizhu-Pengshui and Western Hubei Lichuan-Enshi.The total organic carbon type Mainly dominated kerogen type I, containing also a small amount of kerogen typeⅡ1, the total organic carbon content is higher, but changes in range is larger too, the total organic carbon content of Niu-Titang formation of black shale is 0.04%-14.3%, average 3.42%, Wu-feng formation and Long-ma-xi formation of black shale is 0.07%-7.94%, average 2.54%, the maturity of two sets black shale were generally higher, the maturity of Niu-Titang formation of black shale is 1.29%-5.5%, average 3.01%, Wu-feng formation and Long-ma-xi formation of black shale is 1.04%-4.3%, average 2.59%, the most are in the stage of post mature. 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 O.O1mD, porosity mainly distribute between 2%and 7%, suggesting that the two marine shale has good reservoir conditions. The result of isothermal adsorption simulate test and well site desorption showed that, the gas content of Niu-Titang formation, Wu-feng formation and Long-ma-xi formation of black shale mainly distribute between 1.0m3/t and 3.0m3/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 type and content, total hydrocarbon content, maturity, gas content, fragile mineral content, clay mineral content, porosity, pore character, etc, which are the main controlling factors of shale gas accumulation, these factors has an important influence of shale gas accumulation.Initially establish the forecast model and parameter selection criteria of shale gas development block (potential areas, favorable area and core area), The favorable areas for developing shale gas reservoirs of the two marine shale were by comprehensive information method in the region of interest. The favorable areas of developing shale gas reservoirs of Lower Cambrian are Southwest Sichuan Ziyang-Zigong and Southern Sichuan Yibin-Luzhou-Xishui, Northern Guizhou Cengong-Jiangkou-Songtao, Southwest Chongqing Xiushan, Western Hunan Longshan, Western Hubei Xianfeng-Enshi-Lichuan and Northern Sichuan Wuxi, etc; The most favorable areas for developing shale gas reservoirs of Upper Ordovician and Lower Silurian are Southwest Sichuan Zigong, Southern Sichuan Yibin-Luzhou, Eastern Sichuan Qijiang-Nanchuan and Northern Guizhou Daozhen, Eastern Chongqing Shizhu-Wulong-Pengshui-Qianjiang, Southeast Chongqing Youyang, Northeastern Guizhou Yanhe, Western Hubei Lichuan-Enshi-Yichang-Kaixian and Western Hunan Longshan, etc.