松辽盆地北部二氧化碳气藏成因地球化学研究

GEOCHEMICAL CHARACTERISTIC OF CHANGDEDONG CO_2 GAS RESERVOIR IN THE NORTHERN SONGLIAO BASIN

松辽盆地北部深层已发现的二氧化碳(CO2)工业气藏主要处于徐家围子断陷带深层,火山岩储集层的岩石化学数据表明,徐家围子的火山岩以钙碱性为主,为幔源岩浆分异作用的产物。徐家围子断陷带昌德东气藏天然气中的CO2含量高(89.73%~90.38%),CO2的碳同位素值为-6.61‰^-4.06‰,落在无机成因区,3He/4He值为3.9×10-6和4.5×10-6,介于幔源与壳源之间,伴生甲烷同系物的碳同位素呈倒序排列,具有无机成因气负碳同位素系列的特征。CO2/3He值为1.9×109,指示出气藏中CO2是上地幔脱气成因。火山岩储集层岩石化学数据和气体化学成分判别的结果说明徐家围子断陷带昌德东气藏的形成和幔源岩浆有关,其CO2是无机幔源成因。

Industrial CO2 gas reservoirs discovered in the northern Songliao Basin locate mostly in the deep strata of the Xujiaweizi Fault Depression. The identification of origin of CO2 in the reservoirs would be significant to research the model to form deep gas reservoirs. It was suggested that the volcanic rocks in deep strata of this area were calc-alkaline and differential production of mantle-derived magma by petrochemical data of reservoir rocks. In the Changdedong gas reservoir of Xujiaweizi area, the CO2 contents are 89. 73%- 90.38% and δ^13Cco2 are -6.61‰-4.06‰, which suggested that CO2 is inorganic origin; ^3 He/^4 He ratios are 3.9 × 10^-6 and 4.5 × 10^-6 , which is between mantle-derived and crust-derived origin. There is reverse order of carbon isotopic ratio to accompanying methane homologous compounds, which is characteristic of inorganic origin. The CO2/^3 He ratio of 1.9 × 10^9. indicated that CO2 in this reservoir should be from degassing of upper mantle. The formation of the Changdedong gas reservoir was related to mantlederived magma, and the CO2 gas was inorganic and mantle-derived.