桦甸盆地始新统油页岩稀土元素地球化学特征及其地质意义

Geochemical Characteristics of Eocene Oil Shale and Its Geological Significances in Huadian Basin

稀土元素因其稳定的地球化学性质已经成为研究油页岩等细粒湖泊沉积物的物源、古环境、古气候等变化的良好指示剂,具有重要的研究意义。笔者测试并重点分析了桦甸盆地始新统桦甸组油页岩REE地球化学特征。分析结果表明:桦甸含油页岩段稀土元素含量要低于上下段泥岩和炭质页岩样品,油页岩稀土元素总量(∑REE)为(91.51～355.98)×10-6,平均值为162.56×10-6,高于全球平均大陆上地壳成分(UCC),略低于北美页岩;∑LREE/∑HREE及(La/Yb)N等化学参数表明轻、重稀土元素分异明显,REE分布模式为明显的轻稀土元素富集、重稀土元素亏损呈平坦状、具中等程度的负Eu异常和弱负Ce异常。以Ce异常值反映水介质的氧化还原性,桦甸油页岩形成于缺氧的还原环境;以REE的分异程度来表征油页岩的沉积速率,桦甸组自下而上(La/Yb)N值呈逐渐由增大→降低→增大的变化趋势,含油页岩段的沉积速率普遍比下段和上段低,与根据恢复压实后的桦甸组地层厚度和孢粉年龄估算得出的沉积速率结果相吻合,说明较低的沉积速率更有利于有机质的聚集和保存;以∑REE的纵向变化作为古气候波动的代用指标,高∑REE值指示暖湿的气候环境,低∑REE值指示冷干的气候环境。桦甸油页岩形成于干旱与温湿交替的气候环境,气候通过影响湖泊水体蒸发量与补给量的平衡而控制着湖平面的变化,从而控制了油页岩的层数和厚度。

Rare earth elements （REE） with stable geochemical characteristics are good indicators for studying the provenance, paleoenvironment and paleoclimate of fine lake sediments such as oil shale, etc. and have important research significance. The authors test and focus on analyzing the REE geochemical characteristics of Eocene oil shale in Huadian basin, the results show that the contents of rare earth elements in oil shale member of Huadian Formation are lower than that of the mudstone and carbonaceous shale in the upper and lower members, the total content of rare earth element （~,REE） of oil shale varies from 91.51 X 10-6 to 355.98 X 10-6 with an average at 162.56 X 10 6, which is higher than the global average content of continental crust （UCC） and lower than North American shale. The geochemical parameters such as the ratios of ~LREE/~HREE, （La/Yb）N indicate an obvious differentiation of ~,LREE and ~HREE. The REE distribution patterns are characterized by rich in LREE and middle negative Eu abnormity and weak negative Ce abnormity. The Ce abnormity can be used to reveal redox of lake water, Huadian oil shale formed in hypoxic reducing environment. The differentiation degree of REE can be used to reveal the sedimentary rate of oil shale. （La/Yb）N of Huadian Formation varies from high to low to high from bottom to up, the sedimentary rates of oil shale member （E2_3h2） are generally lower than the upper and lower members, the results is consistent with the sedimentary rates calculated by the stratum thickness after recovered compaction and pollen ages, which indicates that lower sedimentary rate is beneficial for the enrichment and conversation of organic matters. The vertical change of ~ REE can be used as substitute index to reflect the fluctuation of paleoclimate. Usually, high ~REE indicate warm and humid climate and low ~REE indicate cold and arid climate. Huadian oil shale formed in an interchangeable climate with warm-humid and cold-arid, the climate controls the change of lake level by influencing the balance between the evaporation and rainfall of the lake water, and then controls the layer numbers and thickness of oil shale.