鄂尔多斯盆地延长组地震前积反射的地质意义

Progradational reflection from lacustrine Yanchang Formation in Ordos Basin,China

在鄂尔多斯盆地三叠系延长组地震剖面上发现了大量的北东、南西和南东方向的前积反射。这些前积反射解释为形成于长7—长6时期、长4＋5—长2时期、长7—长3和长6—长4＋5时期的前积沉积体,每个前积沉积体由多个期次的前积沉积楔组成,岩性为高速的砂、泥岩互层。前积楔之间往往为5~20m厚的低速页岩,它是形成前积体内部斜交强反射的主要原因。根据前积反射可计算出延长组沉积时期古湖盆的最大古水深为228m,最大古坡度为1.7°,湖盆西南陡坡带坡度为0.6°~1.3°,东北缓坡带坡度为0.5°~0.6°。通过地震剖面地震相、连井剖面沉积相的解释和分析,认为延长组沉积时期在浅—深湖盆相带中发育水下分流河道、河口坝、重力流水道等沉积微相,不同微相砂体宽度和宽厚比不同,横向上多彼此连接,前积体内部砂体含量为30%~60%,单砂体在前积反射内部往往呈短轴状、中弱振幅地震相。前积反射区是形成有利储集相带和大型油田的重要区域。

Numerous progradational reflections from the lacustrine Yanchang Formation in the Ordos Basin are identified on 2Dseismic sections.These reflections suggest deltaic and gravity depositions that flowed into the lake respectively from Northeastern,Southwestern and Southeastern directions during the periods of Chang 7to Chang 6,Chang4+5to Chang 2,Chang 7to Chang 3and Chang 6to Chang 4+5.Each of the Progradational body consists of a few progradational lobes of high velocity sandstones and mudstones.Different progradational lobes are divided by low velocity shale with the thickness of 5to 20 meters,which causes a high amplitude oblique event within the progradational body.A maximum water depth up to 228 m and the maximum bed form dip of 1.7degree of the ancient lake can be deducted from the horizontal and vertical distances between the top lap point and down lap point of an oblique event.The southwest slope of the lake was steeper with 0.6-1.3degrees and the northeast slope of the lake was relatively gentle with 0.5-0.6degrees.Great amounts of log and core data of wells located on the progradational reflection sections indicate that the progradations occurred in a background of shallow to deep lakes and their interior sand bodies are made mainly from underwater distributary channels,mouth bars,gravity-flow channel sand body and soon.Different micro facies of sand bodies are of different dimensions and width/thickness ratios.The sandstones in the progradational reflections approximately occupy 30% to 60% of total stratigraphic thickness,and tend to be connected laterally each other.An individual sand body often causes a short axis of medium-weak-amplitude seismic facies.The progradational reflections are significant favorable reservoir facies and many large oil fields have been found within them.