Characteristics and geological significance of canister desorption gas from the Lower Cambrian Shuijingtuo Formation shale in Yichang area,Middle Yangtze region
Luo Shengyuan1, Chen Xiaohong1, Liu An1, Li Hai1, Sun Chong2
1. Wuhan Center, China Geological Survey, Hubei Wuhan 430205, China;
2. Petroleum Geologic Test Center, Petroleum Exploration and Development Research Institute, Sinopec Jianghan Oilfield Company, Hubei Wuhan 430223, China
As displaying good gas reserves, the Lower Cambrian Shuijingtuo Formation shale in Yichang area, Middle Yangtze region is a new exploration area of shale gas in Sichuan Basin. Based on the onsite analysis of gas-bearing property for 64 pieces of Shuijingtuo Formation gas shale from two shale gas exploration wells, this paper determines the analytical gas content, gas composition, carbon and hydrogen stable isotopic composition of analytical gas, analyzes the changes in the analytical gas composition and the gaseous hydrocarbon and carbon dioxide isotope during analytical process, and explores the occurrence state of shale gas, inversion characteristics and geological significance of gas stable isotope. The results show that Shuijingtuo Formation shale contains the analytical gas of 0.32-5.48 m3/t, and the formation with continuous gas content greater than 2 m3/t is 44.05 m thick, demonstrating a strong positive correlation between gas content and TOC. The analytical gas contains methane and ethane of 81.90% -95.48% and 0.78% -3.95%, respectively and also a trace of propane, classified as a typical dry gas; the content of nitrogen is a bit high in non-hydrocarbon gases, with an average of about 6.7%, and the carbon dioxide content is less than 1%, free of H2S. The adsorption gas and free gas accounts for 50% -60% and 40% -50% respectively. In the early stage of adsorption, CH4 and N2, with weak adsorbability, are first desorbed, followed by C2H6 and CO2, with strong adsorbability; until the end of gas analysis, a considerable amount of C2H6 and CO2 still has not been desorbed. During the analytical process, the carbon and hydrogen isotopes have changed, the variation range of δ13CH4, δ13C2H6, δ13C3H8, δ13CO2, δDCH4 and δDC2H6 is -39.92 ‰--25.86 ‰, -41.57 ‰--39.34 ‰, -40.89 ‰--35.46 ‰, -23.42 ‰--19.23 ‰, -136.90 ‰--128.00 ‰ and -160.45 ‰--155.30 ‰, respectively. Due to the mass fractionation effect of isotope, the methane carbon isotope and methane hydrogen isotope remained in the analytical process is increased by 5.15 ‰ -13.33 ‰ and 1.64 ‰ -8.90 ‰ respectively; the ethane carbon, hydrogen isotope and carbon isotope of carbon dioxide is basically unchanged. The gaseous carbon isotope fractionation is also affected by the physical properties of shale, and the larger pore volume will lead to a more significant methane carbon isotope fractionation effect, as well as a difference in the volume content of ethane. The isotope value of the gas sample taken by desorbing the half volume represents the average value of all gases; the average value of δ13CH4, δ13C2H6 and δ13C3H8 is -33.19 ‰, -40.04 ‰ and -39.07 ‰, respectively; the shale gas shows the "reverse order" feature of isotope:δ13CH4 > δ13C2H6,δ13C2H6 < δ13C3H8, δDCH4 > δDC2H6. Similar to the Lower Cambrian Qizhusi Formation in Weiyuan, Shuijingtuo Formation shale gas in Yichang area is also in the early stage of reversal of gaseous hydrocarbon isotope, and has the characteristics of multi-source composite thermogenic gas.
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