鄂尔多斯盆地延安地区下古生界天然气气源分析

Analysis of natural gas source of Lower Paleozoic in Yan’an Area,Ordos Basin

通过对鄂尔多斯盆地延安地区下古生界气源层精细识别与地化参数分析,确定烃源岩主要发育于碳酸盐岩-膏盐岩沉积组合中。含藻类或含泥质碳酸盐岩、泥岩可形成有效烃源岩,平均TOC>2.50%,烃源岩累计厚度大于70 m。延安地区下古生界奥陶系马家沟组天然气组分分析表明,甲烷平均含量为91.61%,含少量重烃、二氧化碳、氮气、硫化氢、氢气及氦气,干燥系数大于95%,为干气气藏。δ^(13)C_(1)为-35.6‰~-29.4‰,δ^(13)C_(2)为-37.8‰~-29.2‰,δ^(13)C_(3)为-33.3‰~-26.2‰,具有油型气特征,表明气源主要来自海相烃源岩。部分天然气甲烷和乙烷碳同位素系列倒转,推断天然气可能为不同层段海相烃源岩生成气或同一层段海相烃源岩在不同成熟阶段生成气的混合。

Through the fine identification and geochemical parameter analysis of the Lower Paleozoic gas source beds in Yan’an Area of Ordos Basin,it is determined that the hydrocarbon source rocks are mainly developed in the carbonate-gypsum rock sedimentary assemblage.Algae bearing or argillaceous carbonate rocks and mudstones can form effective hydrocarbon source rocks,with an average TOC>2.50%,the cumulative thickness of the source rocks is greater than 70 meters.The analysis of natural gas components in Majiagou Formation of Lower Paleozoic Ordovician in Yan’an Area shows that the average content of methane is 91.61%,containing a small amount of heavy hydrocarbons,carbon dioxide,nitrogen,hydrogen sulfide,hydrogen and helium,the drying coefficient is greater than 95%,so it is a dry gas reservoir.The δ^(13)C_(1) is -35.6‰to -29.4‰,δ^(13)C_(2) is -37.8‰ to -29.2‰,and δ^(13)C_(3) is -33.3‰ to -26.2‰,which has the characteristics of oil type gas,indicating that the gas source is mainly from marine hydrocarbon source rocks.The carbon isotope series of methane and ethane in some natural gas are reversed,and it is inferred that the natural gas may be the gas generated from marine source rocks of different intervals or the mixture of gas generated from marine source rocks of the same interval at different maturity stages.