烃源岩作为铀源岩的可能性:研究现状与展望

Can hydrocarbon source rock be uranium source rock?—a review and prospectives

烃源岩与砂岩型铀矿通常同盆共生,除了提供矿化剂之外,烃源岩能否成为铀源岩对砂岩型铀矿的勘探范围向盆地纵深部位拓展具有重要意义。研究针对烃源岩能否成为铀源岩所涉及的三个关键问题,即“铀从烃源岩中迁出的比例、如何随地层流体运移、在何种条件下沉淀和聚集”,梳理了国内外相关研究进展,指出了有必要加强研究的薄弱环节。结果表明:热模拟实验证实烃源岩中的铀能够迁出,迁出的铀很可能以U(IV)/U(VI)混合的形式随含烃地层水和石油运移,温度、压力的降低以及pH、Eh变化会导致铀溶解度的下降和铀运移载体的分解而发生铀沉淀,沉淀物也可能重新被含氧的地层水溶解。问题与建议包括:(1)铀从烃源岩中迁出的比例存在不确定性,迁出的机制以及地质规律尚不清楚,需要开展进一步的生烃排铀模拟实验及排铀动力学表征研究;(2)铀在低温、含烃、还原性热液中的赋存状态是研究其迁移机制的基础,目前对与铀结合的优势配体的类型、产物热力学性质、铀在含烃地下水与石油中的分布比例所知甚少,有必要开展基于热模拟实验的原位测试研究;(3)携铀流体向浅部运移的过程中温度、压力、pH、Eh、有机无机组分的变化控制铀的迁移/沉淀,不同组合条件下铀赋存形式的转化规律、主控因素尚不清楚,有待开展多因素、多变量的烃铀运移模拟实验进行揭示。

The coexistence of hydrocarbon source rocks and sandstone-hosted uranium(U)deposits in the same basin has been widely reported.Hydrocarbon source rock contributes to uranium precipitation and enrichment by providing oil and gas;whereas,whether it can be a source of uranium supply is of great relevance as to weather uranium prospecting should expand deep into the basin.This study reviews relevant domestic and international studies and offers perspectives on this topic,focusing on three key issues:migration potential of uranium in source rock,possible ways of uranium transport by formation fluids,and mechanisms of uranium precipitation and accumulation.Hydrothermal modeling results show that uranium migration can occur during hydrocarbon generation and expulsion,and the migrated-uranium,probably a mix of U(IV)and U(VI),is likely transported by both hydrocarbon-bearing formation water and oils.The transported-uranium precipitates due to a decrease of uranium solubility and decomposition of the transport fluids caused by a decrease of temperature and pressure and change of pH and Eh;the uranium precipitates can also redissolve in oxygen-rich formation water.The main perspectives are:(1)the amount of migrated-uranium is uncertain,and the mechanism and laws of U migration are still unclear,thus further modeling experiments on source rock-uranium system is recommended to understand the kinetics of uranium migration.(2)Up to now,little is known about the dominant U mobile forms and their thermodynamic properties and distribution between hydrocarbon-bearing formation water and oil,thus,in-situ thermal testing via thermal simulation experiments is recommended to address this issue.(3)During uranium upward transport,changes in temperature,pressure,Eh,and organic/inorganic components of the transport fluids control uranium geochemistry,thereby,in order to understand uranium geochemistry and its controlling factors under different conditions,multi-variable simulation experiments on hydrocarbon-uranium transport is suggested.