景谷盆地低演化油气的同位素地球化学特征

Isotopic Geochemical Characteristics of Low Mature Petroleum in Jinggu Basin

对云南景谷盆地第三系原油及其伴生气的研究 ,共采集了油样 6个 ,气样 4个 ,进行了原油及其族组成、气体甲烷及其同系物的碳同位素测定 ,结果表明景谷盆地大牛圈原油富集轻碳同位素 ,其δ13 C1值为 - 32 .2‰～- 31.0‰ ,族组成碳同位素也偏轻 ,并具有δ13 C饱 <δ13 C芳 <δ13 C非 >δ13 C沥异常分布的类型曲线 ,各组成之间同位素组成的相对差值较小。这些都反映出原油主要来自湖相烃源岩 ,形成于低演化阶段的未熟—低熟原油。原油伴生气甲烷明显富集δ12 C ,δ13 C1值 - 5 7.8‰～ - 5 3.8‰ ,用低演化阶段油型气δ13 C1与Ro 模式计算景谷盆地烃源岩的成熟度约为Ro=0 .3%～ 0 .5 %,与景谷盆地地质背景基本一致。伴生气乙烷碳同位素偏重 ,甲烷同系物碳同位素出现反序列现象 ,可能有煤成气贡献的多源复合。上述特征表明 ,它是与盆地原油同型、同阶段的产物 ,即油为湖相烃源岩形成的未熟—低熟原油 ,气为同一演化阶段形成的油型气 ,其阶段和地球化学特征与天然气的“过渡带”气相当 ,因此主源为低演化阶段形成的油型气。

We researched the Tertiary oil and associated gas in the Jinggu basin in Yunnan. We gathered 6 oil samples and 4 gas samples, and measured the carbon isotopes of oils, oil fractions, methane and its homologues. The results showed that the oils from the Daniuquan in the basin are enriched in δ 12 C, the δ 13 C values range from 32.2‰ to 31.0‰, the group compositions are also light in carbon isotopes with an normal type curve of δ 13 C Sa .<δ 13 C Aro .<δ 13 C Nh .>δ 13 C Asph . and the relative difference of the carbon isotopes of group compositions is also small. These show that the oils were derived from lacustrine source rocks and were formed at a stage of immaturity to low maturity and the associated methane is obviously enriched in δ 12 C with δ 13 C 1 ranging from -57.8‰ to-53.8‰. Calculated by a δ 13 C-R o formula of low mature oil-type gas, the Ro values of the source rocks change from 0.3% to 0.5% , coincident with the geological backgrounds of the basin. The associated ethane is also enriched in δ 12 C, the carbon isotopes of methane homologues show a reversed sequence,i.e., the δ 13 C values of them get positive with the increase of their carbon number, which may be an indicator of mixture of coal-type gas. The characteristics above show the gases were from by the same type of source rocks at the same stage as the oils, i.e., the oils and gasses were both generated by immature and low mature lacustrine source rocks, the evolutionary stage and geochemical characteristics of the gases are equivalent to Biothermal Catalysis Transitional Zone gas.