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至2003年底,中国发现6个千亿立方米以上储量规模的大气田,其中5个在鄂尔多斯盆地(苏里格、乌审旗、榆林、大牛地、靖边),1个在塔里木盆地(克拉2)。根据150个气样的组分、143个气样烷烃碳同位素和21个气样氦同位素的分析数据,这些大气田天然气具有以下地球化学特征①高含烷烃气,低含二氧化碳。烷烃气含量均在90%以上,多数在95%以上,二氧化碳含量基本低于3%,主要在1.5%之下。②烷烃碳同位素组成重,具有煤成气特征。δ13C1值为-38.5‰～-26.2‰,主峰值为-35‰～-32‰;δ13C2值为-35.3‰～-17.8‰,一般为-28‰～-24‰;δ13C3值为-29.9‰～-19.1‰,一般为-27‰～-23‰;δ13C4值为-25.6‰～-20.3‰,一般为-23.5‰～-22‰;δ13CiC4>δ13CnC4。大气田气源岩是石炭系-二叠系和中、下侏罗统煤系。③3He/4He值为n×10-7～n×10-8,具壳源氦特征。CH4/3He为n×1010～n×1011,说明CH4为有机成因。图5表6参41
By the end of 2003, China had discovered six large gas fields with reserves of over one hundred billion cubic meters, of which five were in the Ordos Basin (Sulige, Wushen Banner, Yulin, Daniudi and Jingbian) and one in the Tarim Basin Carat 2). Based on the data of 150 gas samples, 143 gas samples of paraffin carbon isotopes and 21 gas samples of helium isotopes, these gas reservoirs have the following geochemical characteristics: ① high alkane gas content and low carbon dioxide content. Alkane gas content of more than 90%, most of 95%, carbon dioxide content of less than 3%, mainly below 1.5%. ② alkane carbon isotope composition, with coal gas characteristics. δ13C1 values of -38.5 ‰ ~ -26.2 ‰, the main peak of -35 ‰ ~ -32 ‰; δ13C2 value of -35.3 ‰ ~ -17.8 ‰, generally -28 ‰ ~ -24 ‰; δ13C3 value of -29.9 ‰ ~ -19.1 ‰, generally -27 ‰ ~ -23 ‰; δ13C4 value of -25.6 ‰ ~ -20.3 ‰, generally -23.5 ‰ ~ -22 ‰; δ13CiC4> δ13CnC4. Gas source rocks in the gas field are Carboniferous-Permian and Middle-Lower Jurassic coal measures. ③ 3He / 4He value of n × 10-7 ~ n × 10-8, with shell-source helium features. CH4 / 3He is n × 1010 ~ n × 1011, indicating that CH4 is an organic cause. Figure 5 Table 6 参 41