RESUMO
The Sichuan Basin in southern China is well-known for its large natural gas resource potential stored in Sinian-Cambrian systems. Recently, high-yield industrial gas flow has been discovered from the Dengying Formation (Sinian System) and Canglangpu Formation (Cambrian System) in the Penglai gas area, preluding the multilayer stereoscopic exploration in Sichuan Basin. However, the origin of the natural gas and its preserving mechanics is still debated, and thus, in this study the geochemical characteristics of the natural gas are systematically analyzed, based on the data from gas composition and hydrocarbon isotope of a series of local wells. On this basis, the geochemical characteristics of natural gas in different regions and layers are compared, and the reasons for these differences from the origin and influencing factors are analyzed. The results show the following: (1) The natural gas of the Penglai gas field is dry gas dominated by CH4, and the Sinian Dengying Formation gas has lower C2H6 content, larger dryness coefficient, heavier δ13C, and lighter δ2HCH4 than the Cambrian gas, which is associated with the high proportion of hydrocarbons from the high-maturity Dengying source rocks. (2) The natural gas from some wells in the lower part of the structure is characterized by high H2S content and low CH4 content, and heavy δ13C in the components, which seems to be affected by the thermochemical sulfate reduction (TSR) effect. (3) The natural gas from the Penglai gas area has a relatively low maturity, which appears to be attributed to the continuous sealing ability of the caprock, which can preserve both the early generated gas and the late thermal-cracked gas.
RESUMO
This study used Tenax TA absorption tubes to sample volatile aromatic compounds from different emission sources and functional zones in Taiyuan City, Shanxi Province, China. Thermal desorption-gas chromatography-isotope ratio mass spectrometry (TD-GC-IRMS) was subsequently employed to analyze the stable carbon isotope characteristics of the volatile aromatic compounds. The results revealed that the stable carbon isotope ratio (δ13C) of the volatile aromatic compounds emitted through diesel, gasoline, and solvent volatilization, vehicle exhaust, and domestic coal combustion ranged from (-30.79±0.98) to (-29.10±0.14), (-30.96±0.88) to (-28.02±1.77), (-32.13±0.59) to (-27.67±0.49), (-27.58±0.16) to (-25.50±0.75), and (-25.14±0.93) to (-23.44±1.32), respectively. The δ13C value of styrene was (-23.44±1.32), which was only detected in the fumes emitted through domestic coal combustion. Additionally, the sample analysis based on data collected from four different functional zones of Taiyuan City revealed the following:â the δ13C values of the atmospheric volatile aromatic compounds in the mixed residential and traffic zone ranged from (-25.61±2.20) to (-23.91±0.78). Compared with other functional zones, the emissions in this zone were enriched with13C; and â¡ the δ13C values measured in the industrial zone ranged from (-29.15±1.06) to (-24.53±1.07); the emissions in this functional zone were relatively low in 13C compared with other zones. A comparison of the δ13C values of the atmospheric volatile aromatic compounds and emission sources indicated that the main sources of volatile aromatic compounds at the four sampling points in Taiyuan were vehicle exhausts and domestic coal combustion, while the air sampled in the industrial functional zone was heavily affected by the volatilization of solvents.
