鄂尔多斯盆地南部旬邑地区延长组砂岩地球化学特征

Geochemical characteristics of the sandstones of the Yanchang Formation in the Xunyi area,southern Ordos Basin

通过对鄂尔多斯盆地南部旬邑地区延长组砂岩进行主量元素、微量元素和稀土元素分析,明确了旬邑地区延长组沉积期的沉积环境、源区风化程度以及古气候条件等,进一步探讨了研究区砂岩的源区母岩岩性及构造背景等。微量元素比值及其相关图解表明,鄂尔多斯盆地南部地区延长组沉积时期属于氧化环境,能够反映古气候条件的重要指标均显示旬邑地区延长组沉积期属于典型的干旱型气候。砂岩样品的化学蚀变指数(CIA)值分布于10.93~86.17之间,平均值为45.38,化学风化指数(CIW)值分布于0.42~56.42之间,平均值为10.52,表明研究区砂岩从源区搬运至沉积盆地的过程中经历的风化作用较弱,该结论与A—CN—K三角图解所得出的结论相一致,而且Th—U图解与Sm—Ce图解表明沉积物经历了简单的沉积循环过程。砂岩样品稀土元素球粒陨石标准化模式总体显示轻稀土(LREE)相对富集,重稀土(HREE)相对平坦,铕具有“V”形负异常,铈异常不明显,曲线表现为明显右倾,与UCC和PAAS的分布形态类似,表明旬邑地区延长组砂岩物源来自上地壳酸性母岩区,该结论与F1—F2图解、Cr/V—Y/Ni图解、Th/Co—La/Sc图解、Th/Sc—Zr/Sc图解及La/Yb—ΣREEs图解所得出的结论相一致。La—Th—Sc与Th—Sc—Zr/10图解表明研究区砂岩的源区大地构造背景属于大陆岛弧和活动大陆边缘环境。显著高的Fe、P等元素含量和较高的烧失量显示延长组沉积期富含营养物质,而且干旱炎热的气候条件和风化程度较低的酸性火成岩有利于研究区烃源岩有机质的保存和转化,有利的储层条件和优势储盖组合为研究区发育大型油气田提供了良好的条件。

The major elements, trace elements and rare earth elements of the sandstones of Yanchang Formation in the Xunyi area from the southern Ordos Basin were analyzed to research the sedimentary environment, the weathering intensity of the source area and the paleoclimate, as well as to decipher the source rock type and tectonic setting of these sandstones. Trace element ratios and the related diagrams suggest that the depositional environment for the Yanchang Formation in the Xunyi area of the southern Ordos Basin belong to oxygenation environment. The important indicators that can reflect the paleoclimatic conditions show that during the Yanchang Formation in Xunyi area it belonged to a typical arid climate. The chemical alteration index(CIA) value of the sandstone ranges from 10.93 to 86.17 with an average of 45.38, and the chemical Index of Weathering(CIW) value of the samples ranges from 0.42 to 56.42, with a mean value of 10.52, which reflect that the sandstones experienced weak weathering since it was carried from source area to the sedimentary basin, which is consistent with the conclusion of A-CN-K triangle diagram. The diagrams of Th-U and Sm-Ce show that the studied sandstones are first-cycle deposits. The chondrite-normalized REE patterns of the studied sandstones show a slight LREE enrichment, smooth HREE slopes, a negative Eu anomaly in a “V” shape and Ce is normal. The REE patterns are characterized by obvious “right” and are comparable to UCC and PAAS, these indicate that the sandstones being derived from the upper continental crust mainly composed of felsic igneous source rocks.This conclusion is consistent with the conclusions obtained from the diagrams of F1-F2, Cr/V-Y/Ni, Th/CoLa/Sc, Th/Sc-Zr/Sc and La/Yb-∑ REEs. Ternary diagrams of La-Th-Sc and Th-Sc-Zr/10 of the sandstone demonstrate the depositional tectonic setting of their parent rocks evolved in a continental island arc or an active continental margin setting. The significantly high content of Fe, P and other elements and the high loss on ignition indicate that the sediments was rich in nutrients during Yanchang Formation. Moreover, the arid and hot climate conditions and the acid igneous rocks with low weathering degree are conducive to the preservation and transformation of organic matter in the hydrocarbon source rocks in this area. The favorable reservoir conditions and dominant reservoir cap assemblages provide good conditions for the development of large oil and gas fields in this area.