四川盆地中二叠统栖霞组白云石化海相流体的地球化学依据

四川盆地中二叠统栖霞组是油气勘探的重点层位,为探究其白云石化流体的性质和来源,基于岩石学研究成果,对该区栖霞组的宿主方解石和不同类型的白云石进行了稀土元素地球化学特征分析。结果表明:(1)宿主方解石、他形白云石和鞍形白云石具有低的稀土元素含量,且Sc、Ti、Zr、Hf等元素的含量显著低于陆源碎屑沉积物中这些元素的含量,表明栖霞组碳酸盐岩未受陆源碎屑物质影响;(2)他形白云石和鞍形白云石具有轻稀土亏损、重稀土富集、La正异常、Ce亏损、Gd富集、显著的Y异常、Eu弱亏损和相似的REE+Y配分曲线,类似于宿主方解石的稀土元素地球化学特征,这些特征表明他形白云石和鞍形白云石的白云石化流体主要来源于地层中的海水或海源流体;(3)个别鞍形白云石样品微弱的Eu正异常及较高的包裹体均一化温度表明,栖霞组受到与峨眉山大火成岩省有关的热事件影响,但热液流体对白云石形成的影响程度相对有限,可能更多的是热事件的温度传导效应使得栖霞组的宿主方解石克服了白云石形成的动力学屏障。结论认为:高温仅是栖霞组宿主方解石白云石化的诱导因素,而白云石化流体仍然主要来源于地层中的海水或者海源流体。

Geochemical characteristics of Middle Permian Qixia Fm dolomitized marine fluids in the Sichuan Basin

The Middle Permian Qixia Fm in the Sichuan Basin is a major target for oil and gas exploration. In order to figure out the properties and source of its dolomitized fluids, it is necessary to analyze the rare earth elements (REE) in host calcite and various dolomites of the Qixia Fm in terms of their geochemical characteristics based on the petrologic research results. The following results were obtained. First, the REE content of host calcite, xenomorphic dolomite and saddle dolomite is low and their Sc, Ti, Zr and Hf content is significantly lower than that of terrigenous clastic sediments, which indicates that the Qixia Fm carbonates were not affected by terrigenous clastics. Second, xenomorphic dolomite and saddle dolomite are characterized by depleted LREE, enriched HREE, positive La anomaly, negative Ce anomaly, enriched Gd, significantly positive Y anomaly, slightly negative Eu anomaly and similar REE+Y partition curve, which are similar to the REE geochemical characteristics of host calcite. It is therefore demonstrated that the dolomitized fluids in xenomorphic dolomite and saddle dolomite are sourced from the sea water or marine-sourced fluid in the formation. And third, some samples of saddle dolomite present the characteristics of slightly positive Eu anomaly and higher fluid inclusion homogenization temperature. It is indicated that the Qixia Fm was affected by the thermal events related to the Emeishan large igneous province, but the influence of hydrothermal fluid on dolomite formation was relatively limited; more likely, due to the temperature conduction effect related to thermal events, the host calcite of Qixia Fm might overcome the kinetics barrier of dolomite formation. To sum up, high temperature is only the inducing factor for the dolomitization of Qixia Fm host calcite, and the dolomitized fluids are still mainly derived from the sea water or marine-sourced fluid in the formation.