塔里木盆地阿克苏地区中上奥陶统烃源岩地球化学特征及沉积环境

Geochemistry Characteristics and Sedimentary Environment of Middle-Upper Ordovician Source Rock of Aksu area in Tarim Basin

塔里木盆地西部中上奥陶统萨尔干组页岩和印干组页岩是古生代重要的烃源岩,该盆地西部中上奥陶统烃源岩的研究对于探寻油气成藏有利区带及指导实际勘探有重要意义。为进一步分析其形成时的环境条件,应用地球化学手段对其进行研究,选取露头上具有代表性的样品对其进行微量元素和稀土元素分析、岩石热解分析和矿物含量分析。结果表明,该烃源岩的微量元素分配模式以U、Nd和Sm富集,Ba和Sr亏损为特征,∑REE介于(80.04～370.2)×10-6范围,LREE/HREE值为10.87～13.99,LaN/YbN值为7.92～11.218,δEu为0.680～0.849,稀土配分模式属轻稀土富集型。有机碳(TOC)含量为0.39%～2.93%,氢指数(HI)为39～122mg/g,说明该烃源岩处于较成熟阶段,尚有一定的生烃潜力。综合该地区萨尔干组页岩与印干组页岩的露头岩性特征、富含笔石与腕足类化石以及黄铁矿发育等特点,结合室内微量元素、稀土元素、岩石热解数据以及X射线衍射定量综合分析,认为阿克苏中上奥陶统地层发育于深水陆棚—盆地沉积环境中。

The Middle-Upper Ordovician Salgan and Yingan shales developed in Paleozoic are the important marine hydrocarbon source rocks in western Tarim Basin.Research on the Middle-Upper Ordovician hydrocar bon source rocks is significant to seeking the favorable districts for hydrocarbon accumulation and guiding the practical exploration in western Tarim Basin.We carried out geochemical analysis and explored its depositional environment.We selected representative samples from outcrops for trace elements and rare earth elements analysis,rock pyrolysis analysis and mineral content analysis.The results show that the rock is rich in U,Nd and Sm with a negative Ba and Sr anomaly;∑REE is(80.04-370.2)×10-6,LREE/HREE is 10.87-13.99,LaN/YbN is 7.92-11.218,and δEu is 0.680-0.849.Rare earth assemblage model belongs to LREE enrichment type.The TOC is 0.39%-2.93% and HI is 39-122mg/g,which show that the hydrocarbon source rocks are at the mature stage with some hydrocarbon generating potential.Based on the lithological characteristics of the outcrops of Salgan and Yingan shales samples and features of abounding with graptolite and brachiopod fossils and development of pyrite,and quantitative analysis on trace element,rare earth element,rock pyrolysis data and X-ray diffraction,it is considered that the Middle-Upper Ordovician strata in Aksu area grew in deepwater shelf-basin depositional environment.