塔里木盆地典型油气区下古生界储层油田水的年龄研究：129I/I比值的综合应用

As a result of multiple hydrocarbon generation events and tectonic movements, it is complicated for the source and distribution of hydrocarbon and water in the Lower Paleozoic strata of the Tarim Basin, which has drawn high attention from the geochemists for a long time. The iodine ion in oilfield water can provide the information about the oil and gas sources, while 129I isotope contains the time information on water formation. Integrated analysis of both can be used to not only provide the age of reservoir water to reconstruct the water's evolution model, but also recover the history of the contact of hydrocarbon with water to indicate the hydrocarbon source and to provide the time of the formation and adjustment of the oil and gas accumulation. Therefore, it is critical to develop the precise Iodine-129 dating research on local oilfield water. Preliminary investigations using the 129I dating method have been done on the oilfield water in the Ordovician strata from the Lunnan area. Our results have indicated that the oilfield water in deep reservoirs in the Tarim Basin could be a mixture of expelled brine from the source rocks and the meteoric water after Mesozoic. Based on these work, this study will select the oilfield water in Cambrian and Ordovician reservoirs from the Lunnan and Tazhong area of the Tarim Basin as the research object. Both the iodine concentration and 129I/I isotope ratio of the oilfield water will be measured, and the times of the entering of each end members of oilfield water into the reservoirs can be determined by using the iodine tracing and 129I/I dating methods. This project aims to reconstruct the evolution history of oilfield water and recover the charging process of oil and gas, and finally to provide new evidences to the understanding of the oil and gas sources.

受多期次生烃和构造运动的影响，塔里木盆地下古生界储层的水烃来源和分布异常复杂，长期以来为人们高度关注。油田水中的碘离子可以提供油气的源区信息，而129I同位素则包含着水形成的时间信息。二者的综合分析不仅可以获得储层油田水的年龄，重建油田水的演化模式，而且可以恢复油气与水的接触历史，提供油气来源方向以及油气藏形成与改造的时间信息。因此，开展油田水的碘129定年研究十分重要。申请人对轮南地区奥陶系储层油田水开展了初步的129I定年研究，结果显示深部储层中的油田水可能是烃源岩排出水与较新古大气降水混合的产物。以此为基础，本项目拟以塔里木盆地塔中和轮南地区寒武-奥陶系储层油田水为研究对象，测定其碘离子含量和129I/I比值，采用碘离子示踪和129I定年的研究方法，确定油田水各个端员组分进入储层的时间，并刻画油气充注历史。项目预期能够重建油田水的演化历史，同时为解释油气来源提供新的依据。

做为典型的叠合盆地，塔里木盆地的地质构造运动频繁，地质流体活动十分活跃，油气来源和分布异常复杂。油田水的化学和同位素组成，特别是与有机质有关的碘及其同位素129I，能够提供水和油气来源演化的关键信息。本项目以塔里木盆地塔中和轮南地区奥陶系储层油田水为研究对象，在常规水化学和同位素分析的基础上，重点依靠碘离子示踪油气源区方向和129I示踪流体来源并限定流体年龄的方法，揭示了储层油田水和油气的协同演化历史。研究表明，轮南地区油田水是由中新世（～10Ma）贫碘的古大气降水自潜山高部位向下渗透进入储层，并和白垩纪时来自草湖坳陷烃源岩、伴随原油充注的富碘古海水不均匀混合而成的。塔中地区油田水被精细区分为三个不同期次的古海水端元。第一期的贫碘古蒸发海水是在晚奥陶世从深部非烃源岩的地层向上运移进入储层。第二期富碘的古蒸发海水是在晚古生代随着原油从满加尔坳陷的烃源岩运移而来的。这两期古海水已获得灰岩储层铀裂变生产的、低的129I含量（<35 atom/uL）。第三期的古海水富129I（>100 atom/uL），是在喜山期（晚新近纪）与深部烃源岩生成的天然气一起运移而来的。塔中和轮南地区不同的构造活动造成了两地油田水演化的差异。利用碘离子和碘129定年/示踪的方式，结合水化学和相关油气生产数据，可以有利地揭示了油田水形成演化和油气成藏的密切关系；完全可以通过油田水的地球化学研究，解释储层中油气的成藏历史。