Abstract:

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 91.51×10^-6 to 355.98×10^-6 with an average at 162.56×10^-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 (E_2-3h²) 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.