GEOCHEMICAL ANOMALIES OF THE LOWER SILURIAN SHALE GAS, SICHUAN BASIN, CHINA INSIGHT FROM THE RAYLEIGH-TYPE FRACTIONATION MODEL AND GEOLOGICAL DISTRIBUTION

Unraveling the geological processes creating the geochemical anomalies of the shale gas in the Sichuan Basin has great significance for understanding the mechanism of gas generation in the late stage. Through the first supplement of gas samples in the southwestern part of the Fuling block and the expanded exploration areas in Changning-Zhaotong and Weiyuan, a systematic comparison study including gas composition, stable carbon, hydrogen and nitrogen isotopic composition, and noble gas component was carried out. The results reveal that the Lower Silurian shale gas is characterized by extraordinarily high methane (>96.11%) and low C₂₊ hydrocarbon (<0.51%).The δ¹⁵N_N2 values of the LM shale gases range from −5.2 ‰ to −1.1‰, suggesting the nitrogen is produced from the thermal ammoniation of organic matter. Helium component is typical crustal genesis as its concentration range from 144.8 ppm to 237.5 ppm and its R/R_a range from 0.01 to 0.05. Full reversal (δ¹³C₃< δ¹³C₂< δ¹³C₁) phenomenon and abnormally positive δ¹³C₁ value constitute the carbon-isotope anomalies of the Lower Silurian shale gas.

References

Dai, J., C. Zou, D. Dong, Y. Ni, W. Wu, D. Gong, Y. Wang, S. Huang, J. Huang, C. Fang, D. Liu, 2016, Geochemical characteristics of marine and terrestrial shale gas in China: Marine and Petroleum Geology, v.76, p.44-463, doi: 10.1016/j.marpetgeo.2016.04.027.

Feng, Z., D. Liu, S. Huang, W. Wu, D. Dong, W. Peng, and W. Han, 2016, Carbon isotopic composition of shale gas in the Silurian Longmaxi Formation of the Changning area, Sichuan Basin: Petroleum Exploration and Development, v. 43, n. 5, p.769–777, doi: 10.1016/s1876-3804(16)30092-1.