羌塘盆地南部古油藏带布曲组碳酸盐岩稀土元素特征及意义

Characteristics and indications of rare earth elements in carbonates in the Buqu Formation, southern Qiangtang Basin

万友利 王剑 万方 付修根 王忠伟 沈利军

Wan Youli Wang Jian Wan Fang Fu Xiugen Wang Zhongwei Shen Lijun

中国地质调查局 成都地质调查中心 国土资源部沉积盆地与油气资源重点实验室, 成都 610081 2. 中国地质调查局 成都地质调查中心 沉积与能源地质研究室, 成都 610081 3. 中国地质大学(武汉) 地球科学学院, 武汉 430074

Key Laboratory for Sedimentary Basin and Oil and Gas Resources, Ministry of Land and Resources, Chengdu, Sichuan 610081, China 2. Sedimentary Geology Research and Energy, Chengdu Center of Geological Survey, China Geological Survey, Chengdu, Sichuan 610081, China 3. School of Earth Sciences, China University of Geosciences, Wuhan, Hubei 430074, China

在羌塘盆地南部古油藏带布曲组碳酸盐岩的详细岩石学分析基础上，研究不同类型碳酸盐岩的稀土元素特征，结果表明：①研究区碳酸盐岩包括灰岩、泥—微晶白云岩、（残余）颗粒白云岩、晶粒白云岩及鞍形白云岩；②各类碳酸盐岩均具有较低的稀土元素总量，能够指示沉积或成岩流体的性质，部分鞍形白云岩样品稀土元素总量高于灰岩，预示着可能有外来流体的加入；③不同类型碳酸盐岩稀土总量具有差别，但经太平洋表层海水标准化后，具有相似的稀土元素配分模式，LaN/YbN平均值为2.86，轻微的轻稀土富集和重稀土亏损，随着白云石化程度的增加，轻稀土富集程度变弱，δCe正异常（平均值2.94）和δEu正异常（平均值1.43），这些特征说明布曲组碳酸盐岩沉积于氧化性质的水体中，白云石化流体主要为同期海源地层水，但在烃类充注以后成岩环境变为还原环境，受早白垩世末期拉萨地体与羌塘地块剪刀式碰撞，研究区布曲组地层中有大气淡水加入，再次受到新生代构造热事件影响，形成鞍形白云岩，构造热事件为Eu2+富集提供温度条件。

The REE characteristics of different carbonate rock types were studied based on petrologic analyses of carbonate rocks from the Buqu Formation in ancient oil pools in the southern Qiangtang Basin. The carbonate rocks in the study area were classified into limestones, clay-dolomicrites, (residual) grain dolostones, crystalline dolo-stones and saddle dolomites. All kinds of carbonate rocks have a low REE content (∑REE), and some of the saddle dolomite samples have a higher ∑REE than limestones, indicating the mixing of outside liquids. Different carbonate rocks have different REE contents, but their distribution patterns are similar due to the standardization of surface waters of the Pacific Ocean. The LaN/YbN ratio averages 2.86, with a slight light rare earth concentration and a heavy rare earth loss. Light rare earth concentration degree decreases with the increase of dolomitization. The positive δCe anomaly (average 2.94) and positive δEu anomaly (average 1.43) showed that the Buqu carbonate rocks were deposited in an oxic environment. Dolomitization fluids mainly came from oceanic formation water in the same period. The diagenetic environment changed to reductive after hydrocarbon charging. At the end of Early Cretaceous, the Lhasa and Qiangtang blocks shear collided, helping atmospheric fresh water enter the Buqu Formation. Affected by the Cenozoic tectonic thermal event again, saddle dolomites were formed. Tectonic thermal events provided temperature conditions for Eu2+ concentration.

基础性公益性油气地质调查项目（121201010000161110）、国土资源调查项目（1212011221114）和国家“十二五”重大科技专项（2011ZX05004-001）联合资助。

https://doi.org/10.11781/sysydz201705655