鄂尔多斯盆地伊陕斜坡中南部高热演化地区古生界天然气地球化学研究

Geochemical study of Paleozoic gas reservoirs in a highly mature area on the south-central Yishan slope, Ordos Basin

苏里格南部和高桥高热演化地区古生界天然气具有乙烷等重烃组分含量低,大多数样品的湿气系数小于2%,天然气甲烷碳同位素组成显著偏重,乙烷等重烃组分碳同位素组成变化大等特征。热演化程度更高、甲烷碳同位素更重、湿气系数更低的东南部地区,古生界天然气均显示乙烷等重烃组分碳同位素异常偏轻、碳同位素倒转的特征。通常,当乙烷等重烃组分含量极低的高演化天然气混入少量乙烷等重烃组分含量高、碳同位素组成显著偏轻的天然气就可形成天然气的乙烷等重烃组分碳同位素异常偏轻、碳同位素倒转。早白垩世末盆地抬升阶段初期古地温降至相当于高-过成熟阶段的温度区间时,气源岩和储层中残余的少量液态烃的热裂解生成的天然气混入导致了乙烷等重烃组分碳同位素异常偏轻与碳同位素的倒转。因此,应以甲烷碳同位素组成作为主要指标、乙烷等重烃组分碳同位素组成作为参考指标,结合成藏地质条件进行高演化天然气的成因判识。提出了研究区古生界天然气以煤成气为主,并有一定数量油型气混入的认识。

Paleozoic gases from the South Sulige and Gaoqiao areas in the Ordos Basin are characterized by low contents of heavy hydrocarbon gases, less than 2% of wetness for nearly all of the gas samples, relatively heavy δ^13C1, and large variations in δ^13C2＋. Gas samples from the southeast of the studied area are of higher thermal evolution, heavier δ^13C1 value and lower wetness, and show abnormally light δ^13C2＋ values and carbon isotopic reversal. In theory, only the mixture of highly mature gas with extremely low contents of heavy hydrocarbon gases and a small amount of natural gas with higher contents and lighter carbon isotopic composition of heavy hydrocarbon gases can lead to the occurrence of abnormally light δ^13C2＋ and the carbon isotope reversal between methane and ethane. The mixed gas probably is derived from the thermal cracking of a small amount of residual liquid hydrocarbons contained in the source and reservoir rocks during the initial stage of the uplift movement at the end of Early Cretaceous. Therefore, δ^13C1 should be used as the principal index and δ^13C2＋ values can be used as reference indexes for the genetic identification of highly mature natural gas. In addition, the combination with comprehensive geological and geochemical studies coal-derived gas, mixed with some oil-related gas. suggests that the Paleozoic gas in the studied area is mainly