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采用地球化学方法对龙岗气田长兴组和飞仙关组天然气来源进行了详细研究,天然气干燥系数很高,乙烷等重烃含量极低,非烃气体中除了少量的N_2和CO_2以外,普遍含有H_2S气体。气田范围内长兴组和飞仙关组本身不具生烃能力。天然气地球化学特征与来自志留系和寒武系烃源岩天然气差异很大,也不可能来自志留系和寒武系烃源岩。气藏中普遍含有储层沥青,被认为是原油裂解的产物。因储层沥青与甲烷碳同位素之间没有明显分馏关系,甲烷并非直接来自原油裂解气。气藏中甲烷和乙烷碳同位素组成都很重,为高—过成熟的煤型气,天然气主要来自龙潭组煤系。与川西坳陷高—过成熟须家河组煤系烃源岩生成的天然气相比,龙岗气田甲烷碳同位素组成异常偏重,是由于烃源岩高演化程度和水溶气脱气混入这2个因素叠加而成,并不是目前多数研究者认为的是由TSR作用造成,因为甲烷碳同位素值和H_2S含量没有明显的相关性。
Geochemical methods were used to study the source of natural gas in Changxing Formation and Feixianguan Formation of Longgang Gas Field. The natural gas has a high coefficient of dryness and a very low content of heavy hydrocarbons such as ethane. The non-hydrocarbon gas contains a small amount of N 2 and CO 2, H_2S gas. Changxing Formation and Feixianguan Formation within the gas field do not have hydrocarbon generation capacity per se. The geochemical characteristics of natural gas are very different from natural gas from the Silurian and Cambrian source rocks, and may not come from the Silurian and Cambrian source rocks. Gas reservoirs commonly contain reservoir bitumen and are considered to be the product of crude oil cracking. Because there is no obvious fractionation relationship between reservoir bitumen and methane carbon isotope, methane does not come directly from crude oil pyrolysis gas. The carbon isotope compositions of methane and ethane in the gas reservoirs are very heavy, and are high-over mature coal gas, which is mainly from the coal series of Longtan Formation. Compared with the natural gas formed from the coal measure source rocks of the Xujiahe Formation in the West Sichuan Depression, the carbon isotope composition of the methane in the Longgang gas field is anomalous because of the high evolution of hydrocarbon source rock and the degassing of water-soluble gas It is not currently believed by most researchers that this effect is due to the TSR effect because the carbon isotope values for methane are not significantly related to the H 2 S content.