湖泊细粒沉积特征精细研究：以鄂尔多斯盆地延河剖面长7油层组为例

Detailed Study of Lake Fine Grained Deposition Characteristics:A Case Study from Chang 7 of Yanhe Section,Ordos Basin

野外地质露头为精细刻画沉积体内部结构、建立准确地下地质模型发挥着重要作用。以鄂尔多斯盆地延河剖面长7段为例,采用岩石学、野外露头沉积学方法,详细剖析了湖泊细粒沉积的岩相类型、特征、垂向组合及沉积环境。研究结果表明,延河剖面长7段发育平行层理细砂岩相、流水交错层理细砂岩相、浪成交错层理粉砂岩相、沙纹层理粉砂岩相、变形层理粉砂岩相、水平层理(泥质)粉砂岩相、块状泥岩相、水平层理(砂质)泥岩相、水平纹层页岩相9种岩相类型。在结合区域地质特征基础上,研究认为长7段为远源的曲流河三角洲前缘和浅湖-半深湖沉积,进一步细分出7类沉积单元,其中水下分支河道、支流间湾较为发育,水下天然堤、远砂坝、席状砂发育规模较小,浅湖-半深湖沉积只在长72段下部发育,河口坝基本不发育,仅局部可见。对各沉积单元的垂向分布特征进行深入研究,识别出Ⅰ、Ⅱ、Ⅲ、Ⅳ4类垂向分布形式,其中Ⅰ、Ⅱ组合主要分布在研究区长71、长73亚段,Ⅲ、Ⅳ组合主要分布在研究区长72亚段。剖面相分析表明,长7沉积期整体为一套先变细、再变粗的细粒沉积序列,为曲流河三角洲前缘沉积—浅湖-半深湖沉积—曲流河三角洲前缘沉积。

Field geological outcrops play a significant role in meticulous depiction of the internal structure of sedimentary deposits and establishing accurate underground geological model. This paper takes Chang 7 of Yanhe outcrop section in Ordos basin for instance,adopts the methods of petrology and field outcrop sedimentology,and detailedly analyses the lithofacies type,characteristies,vertical combination and sedimentary environment of lake fine grained sediments. The results show that Chang 7 of Yanhe outcrop section develops 9 types of lithofacies,including parallel bedding fine sandstone facies,flowing water crossbedding fine sandstone facies,wave forming crossbedding fine sandstone facies,ripple cross lamination siltstone facies,deformation bedding siltstone facies,horizontal bedding（ argillaceous） siltstone facies,bulk mudstone facies,horizontal bedding（ sandy） mudstone facies,big wavy-straight laminae shale facies. On the basis of combining regional geological characteristics,this paper regards the Chang 7 as distal sources meandering stream deltaic front and shallow-semi-deep lake sedimentary,further divided into 7 categories sedimentary units,among which underwater distributary channel and bar between branches developed well,subsea natural barrier,distal bar and sheet sand developed small scale,shallow-semi deep lake sedimentary only developed at the lower part of the Chang 72 subsegment,mouth bar developed hardly,only partial visible. Thorough research on the vertical distribution characteristics of every sedimentary unit,4 vertical distribution forms of Ⅰ,Ⅱ,Ⅲ,Ⅳ are recognized. Ⅰ、Ⅱ combination mainly distributed at the research area Chang 71 and Chang 73 subsegments. Ⅲ、Ⅳ combination mainly distributed at the research area Chang 72 subsegments. Section facies analysis shows that during integral Chang 7 deposition period,a fine grained sedimentary sequence of thin to coarse was formed,showing meandering stream deltaic front sedimentary-shallow lake-semi-deep lake sedimentary-meanderiing stream deltaic front sedimentary.