Earth Science Frontiers ›› 2020, Vol. 27 ›› Issue (3): 262-268.DOI: 10.13745/j.esf.sf.2020.4.8

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Quantitative study of shallow natural gas source in tunnels in the Longquanshan structural zone

ZHANG Xiaolin1(), SU Peidong2,*(), SU Shaofan2, MA Yunchang2, YANG Feng3   

  1. 1. China Railway Eryuan Engineering Group Co., Ltd,Chengdu 610000, China
    2. College of Earth Sciences and Technology, Southwest Petroleum University, Chengdu 610500, China
    3. Civil Engineering College, Southwest Jiaotong University, Chengdu 610031, China
  • Received:2019-11-26 Revised:2019-12-25 Online:2020-05-20 Published:2020-05-20
  • Contact: SU Peidong

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Abstract:

The Longquanshan oil-bearing structural belt is at the boundary between the western and central Sichuan oil and gas fields. Its non-coal strata are mainly Jurassic and Cretaceous red layers producing no hydrocarbons. Yet tunnel constructions across the Longquanshan tectonic belt are often confronted with shallow gas hazard. To ascertain the source of shallow natural gas and provide more accurate prediction on gas distribution characteristics in the non-coal strata tunnels, we studied the Chengdu Metro Line 18 Longquanshan Tunnel as an example, using field composition testing, gas chromatography and stable carbon isotopes to quantitatively characterize the source of shallow natural gas in the area. Previous research predicted that the main components of the shallow natural gas were CH4 (57.65%-75.23%), N2, and CO2. We showed that δ13C1, δ13C2 and δ13C3 values were 40‰, 26‰ and 25‰, respectively, by stable isotopic analysis, and the age of gas source rocks was between 225 and 249 Ma from rare gas isotope experiments. Using biomarker compounds we determined the regular sterane ratio to be between 0.90 and 1.07. Large amounts of shallow natural gas, originated from the underlying Triassic Xujiahe Formation, escaped from the Longquanshan tunnel. The deep gas migrated mainly upward through faults and joint fissures. Enrichment in shallow sandstones, joint fissure development areas, and local structural highs all pose danger to tunnel engineering. The Chengdu Metro Line 18 Longquanshan Tunnel lies cross the Longquanshan petroleum structure. The absolute gas emission from the palm face of the tunnel reached 2.13-4.99 m3/min. The distribution of shallow natural gas is controlled by the Longquanyi fault, Wolongsi syncline, Longquanshan anticline, and Ma'anshan fault. The Longquanyi and Ma'anshan faults provide upward migration pathways in the deep Triassic Xujiahe Formation, where the two faults are associated with derived joint and fissure development areas, while the turning section of the Longquanshan anticline is favorable for shallow natural gas accumulation. Together these geological factors contribute to the high gas concentration and high risk for tunnel construction in the area.

Key words: tunnel, shallow natural gas, gas source, Longquanshan oil-bearing structure

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