柴达木盆地西缘古近系源-汇系统U-Pb定年和重矿物分析

Application of U-Pb isotopic dating and heavy mineral assemblage to source-sink system analysis of the Paleogene in western margin of Qaidam Basin

受阿尔金山物源供给影响,柴达木盆地西缘的储层物性总体上差且非均质性强。为了实现有利储层分布预测,亟待开展源-汇系统精细划分。应用野外露头和钻井岩心样品,综合采用碎屑锆石U-Pb同位素定年、重矿物组合分析以及砂体展布分析等方法,开展了研究区古近系下干柴沟组上段的源-汇系统分析。结果表明:研究区可划分为七个泉、狮北、干柴沟、咸水泉和咸东共5个源-汇系统,不同源-汇系统表现出重矿物组合、标志重矿物含量的差异,以及不同的峰值年龄、峰值年龄组合和年龄谱图形样式;物源年龄主要在200~520 Ma之间,母岩以岩浆岩和变质岩为主。受这些物源供给,研究区相应地发育扇三角洲—湖底扇沉积体系,其规模差异较大。受多物源供给的影响,单一分析方法可能会导致源-汇系统划分的不确定性,因此采取多种分析方法相结合,并且互为验证,才能取得比较正确的认识。

The hydrocarbon reservoirs,which formed with the sediment supply of Altun mountain,have overall poor physical properties and strong anisotropy in the western margin of Qaidam Basin.For the purpose of predicting favorable reservoir distribution,it is urgent to carry out detailed division of the source-sink system.Using field outcrop and drilling core samples,comprehensive methods such as detrital zircon U-Pb isotopic dating,heavy mineral assemblages and sand body distribution are employed to conduct source-sink system analysis of the Upper Member of Lower Ganchaigou Formation of Paleogene in western margin of Qaidam Basin.As a result,five source-sink systems named as Qigequan,Shibei,Ganchaigou,Xianshuiquan,and Xiandong,respectively,are identified.They are different in heavy mineral assemblages and symbolic heavy mineral contents,and present different peak ages,peak age combinations and age spectrum patterns from each other.The age of sediment sources is mainly from 200 to 520 Ma,and the parent rocks is mainly composed of magmatic and metamorphic rocks.Affected by the supply of these sources,fan delta-sublacustrine fan systems are developed with greatly-varied scales in the study area correspondingly.It is shown that a single analytical method may lead to uncertainty in the division of source-sink systems.To achieve a more accurate understanding,multiple analytical methods should be combined and mutually validated.