四川盆地超深层天然气地球化学特征

Geochemical characteristics of ultra-deep natural gas in the Sichuan Basin, SW China

对四川盆地38口井超深层(埋深大于6 000 m)天然气组分及其地球化学特征进行分析,以判明超深层天然气成因。四川盆地超深层天然气组分具如下特征:甲烷占绝对优势,含量最高达99.56%,平均86.67%;乙烷含量低,平均为0.13%;几乎没有丙、丁烷,为干气,属过成熟度气。硫化氢含量最高为25.21%,平均为5.45%;烷烃气碳同位素组成为:δ^(13)C_1值从-33.6‰变化至-26.7‰,δ^(13)C_2值从-32.9‰变化至-22.1‰,绝大部分没有倒转而主要为正碳同位素组成系列。烷烃气氢同位素组成为:δD_1值从-156‰变化至-113‰,少量井δD_2值从-103‰变化至-89‰。二氧化碳碳同位素组成为:δ^(13)C_(CO_2)值从-17.2‰变化至1.9‰,绝大部分在0±3‰范围。根据δ^(13)C_1-δ^(13)C_2-δ^(13)C_3鉴别图版,盆地超深层烷烃气除个别井外绝大部分为煤成气。根据二氧化碳成因鉴别图和δ^(13)C_(CO_2)值,判定除个别井外,超深层二氧化碳绝大部分为碳酸盐岩变质成因。龙岗气田和元坝气田超深层硫化氢为非生物还原型(热化学硫酸盐还原成因),双探号井的超深层硫化氢可能为裂解型(硫酸盐热裂解成因)。

The natural gas components and geochemistry of 50 ultra-deep gas wells （buried depth greater than 6000 m） in the Sichuan Basin were analyzed to determine the genesis of ultra-deep natural gas in the basin. The natural gas components of the basin have the following characteristics： Methane has an absolute advantage, which can be up to 99.56% with an average of 86.6%; ethane is low, with an average of 0.13%; there is nearly no propane and butane. So it is dry gas at over-mature thermal stage. The content of H2S can be up to 25.21%, with an average of 5.45%. The alkane gas isotopes are： The carbon isotope varies from -32.3‰ to -26.7‰ for methane and from -32.9‰ to -22.1‰ for ethane. There is nearly no carbon isotopic reversal among methane and its homologues. Hydrogen isotope varies from -156‰ to -114‰ for methane, and from -103‰ to -89‰ for some ethane. The carbon isotope of CO2 varies from -17.2‰ to 1.9‰ and most of them fall within the range of 0±3‰. According to the δ13C1-δ13C2-δ13C3 plot, except some wells, all other ultra-deep gas wells are dominated by coal-derived gas. Based on the CO2 origin distinguishing plot and δ13CCO2, except individual wells, most of the ultra-deep CO2 are of carbonate metamorphic origin. H2S in the ultra-deep layer of Longgang and Yuanba gas fields belongs to thermochemical sulfate reduction （TSR）, while H2S from Well Shuangtan belongs to thermal decomposition of sulfides （TDS）.