柴达木盆地北缘侏罗系沉积环境演变及其石油地质意义

Evolution of the Jurassic sedimentary environment in northern margin of Qaidam Basin and its significance in petroleum geology

柴达木盆地北缘(柴北缘)东段大煤沟剖面侏罗系沉积层序发育完整。下侏罗统岩性以细砾岩、细—粉砂岩与炭质泥岩和页岩的不等厚互层为特征,其中大煤沟组一、二段与辫状河三角洲平原相关的湖沼相炭质泥页岩较发育,多含叶肢介、双壳类和植物等静水环境生物化石,沉积环境为辫状河三角洲—滨浅湖体系;由3个中期基准面升降旋回构成1个长期旋回。中侏罗统厚层河流—冲积相砂砾岩较发育,化石面貌以介形虫和植物为主,沉积环境为扇三角洲—滨浅湖体系,中上部发育较深湖相沉积;由9个中期基准面升降旋回构成1个长期旋回。不同时期和地区的化石与岩相分布特征反映侏罗纪沉积中心由西向东迁移。构造活动和气候是沉积环境的主要影响因素,但前者起重要的控制作用。早侏罗世潮湿气候下的辫状河三角洲平原为柴北缘提供了重要的生烃母质,但中侏罗世中晚期半干旱气候更有利于形成高品质的烃源岩。

The Lower Jurassic lithology at the Dameigou section of Qaidam Basin is characterized by fine-siltstone interbedded with carbargilite and shale. The carbargilite and shale of lake-swamp facies from the braided delta plain are more developed in Members One and Two of the Dameigou Formation, and most of them contain fossil conchostracans, bivalves and plants of a quiet-water environment. The sedimentary environment is a system of braided delta and shore-shallow lake, and three middle-period cycles constitute a long period fluctuation cycle of datum. Thick sandstones and conglomerates of fluvial-alluvial facies are more developed with fossil aspects mostly of ostracodes and plants in the Middle Jurassic, and the sedimentary environment is a system of fan delta and shore-shallow lake with semideep lacustrine facies developed in the middle and upper parts, and a long period fluctuation cycle of datum is constituted by nine middle period ones. Fossils and sedimentary facies reveal the eastward transportation of the depocenter. Structural movement and climate influence the sedimentary environment greatly, and the former is a control factor. The braided delta plain under wet climate in Early Jurassic is the important hydrocarbon-generating site, but the semiarid climate in Middle Jurassic benefit a generation of high quality source rocks.