Oil & Gas Geology ›› 2019, Vol. 40 ›› Issue (5): 999-1010.doi: 10.11743/ogg20190505

• Petroleum Geology • Previous Articles     Next Articles

Geochemical features and genesis of shale gas from the Lower Cambrian Shuijingtuo Formation shale in Yichang block,Middle Yangtze region

Luo Shengyuan, Chen Xiaohong, Liu An, Li Hai   

  1. Wuhan Center of China Geological Survey, Wuhan, Hubei 430205, China
  • Received:2018-04-24 Revised:2019-03-18 Online:2019-10-28 Published:2019-08-17

Abstract: Strong gas shows are discovered in drilling the Lower Cambrian Shuijingtuo Formation shale in Yichang area,Middle Yangtze,and high commercial gas flow was tested following hydraulic fracturing of the Shuijingtuo shale in Well Yiye1 in the study area,which is a prospective area for shale gas exploration in the Lower Cambrian shale outside the Sichuan Basin.Nine shale gas samples were collected after hydraulic fracturing to analyze the geochemistry and origin of shale gas in the Shuijingtuo Formation,following the study of shale gas composition,as well as carbon,hydrogen,and He isotopic compositions.The results show that methane is the main constituent of the shale gas and varies in concentration from 87.17% to 92.75%,the content of ethane ranges from 0.83% to 0.94%,and the propane is in trace amount; the drying coefficient (Cl/Cl-5) is about 0.99,indicating typical dry gas.The content of nitrogen in non-hydrocarbon gases is slightly higher,with an average of 7.73%,the carbon dioxide content is less than 1%,and there is no H2S.The δ13C methane in the Cambrian shale gas varies between -33.8‰ and -33.1‰ in content,the δ13C ethane value ranges from -39.2‰ to -36.0‰,δ13C propane value ranges from -39.4‰ to -38.5‰,and the δ13C value for CO2 ranges from -16.8‰ to -14.6‰.The δDCH4 methane in the shale gas ranges from -133.8‰ to -128.5‰ in content,while the δDC2H6 ethane ranges from -168.0‰ to -146.1‰.The stable carbon isotopic distribution of the shale gas shows a totally reversed distribution pattern with δ13C1 > δ13C2 > δ13C3,and δDCH4 > δDC2H6.The R/Ra ratio of helium in the Shuijingtuo shale gas ranges from 0.04 to 0.08,indicating the crustal origin of the helium.Considering the shale gas compositions and their ratios,as well as the reversed distribution of carbon and hydrogen isotopes at the high-over mature stage in thermal evolution,we believed that the shale gas of the Lower Cambrian Shuijingtuo Formation in Yichang area belongs to the oil-derived gas,featuring an origin of secondary cracking,which is similar to the Qiongzhusi shale gas in Sichuan Basin.

Key words: carbon and hydrogen stable isotope, geochemistry, shale gas, Lower Cambrian Shuijingtuo Formation, Yichang area, Middle Yangtze

CLC Number: