铀的地球化学特征及其在油气勘探中的应用

GEOCHEMICAL CHARACTERISTICS OF URANIUM AND THE APPLICATION TO OIL & GAS EXPLORATION

探讨了沉积盆地内铀迁移富集与油气运移的关系，以及油气藏上覆岩层中铀迁移富集的碟状异常模式。沉积岩中铀迁移富集是Ｕ(４＋)与［ＵＯ２」(２＋)之间的相互转化引起的，即：氧化环境下，Ｕ(４＋)氧化成［ＵＯ２］(２＋)而迁移；还原环境下，［ＵＯ２］(２＋)还原成Ｕ(４＋)而沉淀富集。有机质及其生成的油气是沉积盆地内最普遍的还原剂，特别是油气的运移能力强，是制约沉积岩层中铀迁移富集的主要因素。由于油气的垂向运移或散失作用，在油气散失通道及油气藏上覆岩层中铀有明显的迁移富集现象，因此利用铀迁移富集特点可以确定深部油气藏及油气散失通道的分布区域。该方法不但能有效预测各种类型圈闭油气藏的分布区域，而巳也是评价盖层或断层封闭性的有效手段。由于铀具放射性，可利用γ能谱仪在野外现场快速测定其含量，因而是一种有效的油气勘探新方法。

Relations between the enrichment of Uranium and the migration of hydrocarbon in sedimentary basins were discussed in thispaper. A dish-like anomaly model of Uranium migration and enrichment in the overlying rocks of reservoir was also explored. Insedimentary rocks the migration and enrichment of Uranium are caused by transformation between U^4+ and [UO_2]~2+. U^4+ is oxidized to [UO_2]~2+ for migration in oxidizing environments,while[UO_2]~2+is reduced to U^4+ for precipitation in reducing environments. Organic matter and generated oil & gas are common reductants in sedimentary basins,and the movements of hydrocarbonare major factors controlling the migration and enrichment of Uranium. Vertical movements and dissipation of hydrocarbon resultin obvious enrichment of Uranium along hydrocarbon leaking passages and also in overlying rocks of reservoir. Therefore characteristics of Uranium migration and enrichment can be used to determine distributions of deep reservoirs and hydrocarbon dissipation passages. This method can also be effectively used to evaluate sealing abilities of cap rocks and faults in addition to forecastoccurrences of different reservoir traps. Due to its radioactivity,Uranium content can be easily measured in fields by gamma-rayspectrometer. This is a new useful method for oil & gas exploration.