基于稀土元素地球化学特征的泥岩沉积环境及物源分析——以鄂尔多斯盆地陇东地区长9油层组泥岩为例

The Mudstone Sedimentary Environment and Provenance Analysis Based on the Geochemical Evidence of Rare Earth Elements:Take Chang 9 Oil-bearing Layer in Longdong Area of Ordos Basin as an Example

通过对鄂尔多斯盆地陇东地区长9油层组19件泥岩样品的稀土元素地球化学特征进行分析,结果表明该地区样品稀土元素总量(∑REE)较高,LREE/HREE值、(La/Yb)N值及REE配分模式均显示富集轻稀土元素,重稀土相对亏损,表现为明显"右倾"。分布曲线在轻稀土段斜率较大,而在重稀土段则呈平缓,Eu处出现一个较明显"V"形,存在Eu负异常,Ce处全部为基本正常或轻微负异常。样品Ceanom值总体反映了泥岩沉积时水体为缺氧的还原环境。(La/Yb)N值反映西南部沉积物的沉积速率较北东部和北西部高。结果还显示泥岩母岩物质源自上地壳,北东地区主要物质来源为太古代花岗岩与元古代变质杂岩体,北西地区物源区母岩为鄂尔多斯盆地东北缘—北缘的太古代花岗岩与元古代变质杂岩体组合,西南地区物源区为陇西古陆及南部的祁连—秦岭古陆的沉积岩母岩区。在La/Yb-∑REEE图解中所有样品点均更倾向于沉积岩区,很好地指示了长9油层组接受西南部主物源区沉积岩母岩的沉积物质较多的特点。

The Chang 9oil-bearing layer in Longdong area of Ordos Basin has become a significant sequence for evaluating and superseding in Changqing Oilfield.According to the analysis on rare earth elements of 19 mudstone samples,the results show that the amount of rare earth elements（∑REE）is high.Based on the distribution patterns of LREE/HREE,LaN/YbNand LREE,the LREE is abundant,but HREE is defective,and it is characterized by obvious＂right＂.LREE curve is obviously rightward heeling,while HREE is even,and Eu is negative anomaly in a distinctly＂V＂shape.In addition,the whole of Ce is normal or a slight negative anomaly.Through the characteristics of Ceanom,the mudstone sedimentary water is in an anoxic environment.The value of LaN/YbNreflects that the sediment rate is higher in the southwest than that in the northeast and northwest,meanwhile,it shows that mudstone on the parent rock material is derived from the earth′s crust.Material in the northeast area is from the archean granite and proterozoic metamorphic complex.In the northwest area,material is sourced from the combination of Archean granite and Proterozoic metamorphic complex in the northeastern-northern margin of Ordos Basin.In the southwestern area,material comes from the west Gansu ancient land and southern Qilian-Qinling continental sedimentary rocks.In the diagram of La/Yb-∑REEE,all the sample points are more inclined to sedimentary area,which indicates that most of the material of Chang 9oil-bearing layer in Longdong area received sediments from sedimentary parent rocks in primary provenance area in southwest.