川东北元坝地区雷口坡组天然气来源与成藏分析

摘要

四川盆地雷口坡组已成为一个重要的天然气勘探目的层，有关雷口坡组天然气的来源以及雷口坡组是否具有生气潜力还存在争议。研究表明雷口坡组泥质白云岩与泥质灰岩有机碳含量平均达到0.72％，具有较高的生烃潜力。雷口坡组天然气以烃类气体为主，干燥系数为0.99。不同于其他海相层系天然气，其CO2和N2含量较少，总量＜10％，仅含微量的H2 S；它们的甲烷碳同位素相对较重（-31.5‰～-36.3‰），而乙烷碳同位素较轻，基本上都小于-28‰（-27.7‰～-36.6‰），且部分气呈δ13 C1＞δ13 C2反序分布，属高-过成熟海相油型气。其烷烃气系列碳同位素组成既不同于上覆的须家河组陆相天然气，也有异于下伏飞仙关组-长兴组海相气，具有不同的气源，经与雷口坡组烃源岩轻烃组成的对比，认为其气源主要来自本层位。成藏条件分析表明，雷口坡组天然气的富集主要受烃源岩和储层的发育及分布控制，只有紧邻烃源岩发育的浅滩相白云岩/裂缝型灰岩构成的“岩性圈闭”才能形成天然气的聚集。%Leikoupo Formation has become an important target in natural gas exploration of the Sichuan Basin in recent years.Some disputes are remained on both the natural gas source and its generation potential for the Leikoupo Formation.It is indicated that the mean TOC for the argillaceous marl and dolomite arrived at 0.72%,with a comparative high capacity of gas generation.The natural gases are dominated by hydrocarbon component with a low amount of heavy hydrocarbons (round 1%)and have a dryness index of 0.99 .Unlike the natural gases found in other marine sequence,the natural gases investigated in this paper contain relatively low concentration of no-hydrocarbons (abundances of CO2 and N2 less than 10% and H2 S in trace amount (<0.01%)).The methane is isotopically somewhat heavy (-31.5‰to -36.3‰),whereas the ethane has relatively light values mostly less than -28‰ (-27.7‰ to -36.6‰),with isotopic reversals in partial gas samples,suggesting a post-mature marine gas.Natural gases in Leikoupo Formation are distinguished in isotopic compositions of alkane hydrocarbons from typical terrestrial gas in Xujiahe Formation and marine gas in Feixian-guan-Changxing Formation,indicating a different gas source.In conjunction with the character of hydrocarbon source and distribution of source rocks,the close similarity of light hydrocarbons with marls of Leikoupo Forma-tion indicate that the natural gases in Leikoupo Formation are likely from the source rock within this sequence. Through the integrated investigation for the gas accumulation in the Leikoupo Formation,it is found that the source rock occurrence controlled the gas enrichment.Namely,natural gas was only accumulated in “lithologic traps”(constituted by shoal dolomite and/or fractured limestone)which located near the source rock.