Characteristics of Lower Palaeozoic hydrocarbon source strata and a primary study of the shale gas accumulation in northern Guizhou Province

Abstract: To find out the distribution regularity and organic geochemical characteristics of the hydrocarbon source strata in Niutitang Formation and Wufeng-Longmaxi Formation in northern Guizhou and its adjacent areas in combination with the specific field geological investigation and previous research results, the authors conducted detailed analysis and investigation, and the result shows that the distribution of the hydrocarbon source strata in Niutitang Formation was controlled by the deposition and depression or taphrogenic trough in early Cambrian. Two hydrocarbon generation depressions were formed in northern Guizhou, which are respectively located along Dafang- Qianxi- Qingzhen area and to the east of Tongzi- Suiyang- Meitang area. The thickness of hydrocarbon source rocks (TOC>1.0%) is 80-110 m. The average organic carbon in Niutitang Formation in northern Guizhou is 1.36%-10.01%; the organic carbon isotope is -34.90‰ to -28.19‰, the organic matter is of I type, and Ro is 1.77%-4.53%. The hydrocarbon source rocks with the thickness larger than 20 m in Wufeng- Longmaxi Formation are mainly distributed to the east of Tongzi- Renhuai-JiaoYuan- Xiashui area, with the average organic carbon being 3.0%-5.0%, and the organic carbon isotope being -31.92‰ to -28.97‰, the organic matter is orⅠ~Ⅱ1 type, and the vitrinite reflectance is 1.73%-2.38%. As shown in the comprehensive analysis, the shale gas in Wufeng- Longmaxi Formation is controlled by the distribution of shale containing organic matters under certain preservation conditions. The areas favorable for shale gas exploration in Longmaxi Formation include the positive tectonics with medium buried depth and the broad and gentle syncline areas. The shale gas accumulation in Niutitang Formation is controlled by the thickness of hydrocarbon source rocks, the formation position of paleokarst in the late Sinian System, the quality of physical properties of rocks in Dengying Formation, the lamellation fracture in Niutitang Formation, the preservation conditions and other factors. The thickness of hydrocarbon source rocks in Niutitang Formation in some anticlines of Qianxi depression is larger than 80 m, with the buried depth being 1500-2000 m. The attitude of the strata on two sides of the anticline is gentle, without the development of fractures but with certain preservation conditions, and hence this area is the target area favorable for shale gas exploration in Niutitang Formation.