鄂尔多斯盆地东北缘府谷剖面中二叠统盒8段沉积相特征

Sedimentary Facies Characteristics of He-8 Member of Middle Permian in Fugu Section, the Northeastern Margin of Ordos Basin, China

鄂尔多斯盆地苏里格东部地区中二叠统盒8段为主力产气层,厘清其沉积相特征对指导该地区的天然气勘探具有重要意义。鄂尔多斯盆地东北缘府谷剖面与苏里格东部地区具有相似的物源体系与沉积环境,通过对府谷剖面进行实测,结合剖面岩石颜色、岩石学特征、微量元素特征、岩相特征等相标志分析,对盒8段沉积相进行准确划分并明确其沉积特征,可以弥补苏里格东部地区盒8段研究大部分局限于钻井、地震等方面,缺乏露头沉积相支撑的不足。结果表明:府谷剖面盒8段以灰绿、黄绿色粗—中粒岩屑砂岩为主,结构成熟度中等—较低;微量元素V/(V+Ni)值表明盒8段沉积时期为氧化—弱还原过渡环境;共划分出12种岩相,其中,块状砾岩相(Gm)、槽状交错层理砂岩相(St)、板状交错层理砂岩相(Sp)、平行层理砂岩相(Sh)为河流沉积中的典型岩相,不同岩相组合构成盒8段沉积体两种不同的垂向沉积序列;盒8_下段—盒8■段为辫状河沉积,盒8■段为曲流河沉积;辫状河的垂向沉积序列由滞留沉积(Gm-Gst-Gsp)→心滩(Gst-Sq-Sp-St-Gm-St)→河道充填(Gm-St-Sh-M)→废弃河道(Sh-Fr-Mh-Sh-Fr-Mh)→越岸沉积(Sh-Fr-Mh-Fr-Mh)→泛滥平原(M-Mh-Mc-Mh-M)等6类微相构成了一个二元结构不明显的正粒序沉积序列;曲流河的垂向沉积序列由滞留沉积(Gm)→边滩(Sp-St-Sm-Sp-St-Sh)→河漫滩(M-Mh-Mc(Fr)-Mh-M)等3类微相构成了一个典型二元结构沉积序列。

The He-8 member of Middle Permian in the eastern Sulige area of Ordos Basin is the main gas producing bed, and is of great significance to clarify the sedimentary facies characteristics for guiding the natural gas exploration. There are similar provenance systems and sedimentary environments between the eastern Sulige area and Fugu section in the northeastern margin of Ordos Basin. By measuring Fugu section combined with facies markers such as color, petrology, trace elements and lithofacies characteristics, the sedimentary facies of He-8 member was divided accurately, and the sedimentary characteristics were clarified to make up for the lack of outcrop sedimentary facies support in most of the previous study of He-8 member in the eastern Sulige area, which was limited to drilling and earthquake. The results show that the grayish green and yellowish green coarse to medium grained lithic sandstone are the main ones in He-8 member of Fugu section, and the structure maturity is medium to low;the trace element V/(V+Ni) ratio indicates that the depositional period of He-8 member is a transitional environment of oxidation-weak reduction;there are 12 types of lithofacies, among which Gm, St, Sp, Sh are typical lithofacies in fluvial sediments, and two different vertical sedimentary sequences are formed by different lithofacies combinations;He-8 lower member to He-8 upper-2 member is braided river deposit, and He-8 upper-1 member is meandering river deposit;the vertical sedimentary sequence of braided rivers consists of six types of microfacies, including stranded sediment(Gm-Gst-Gsp), braided river bar(Gst-Sq-Sp-St-Gm-St), channel filling(Gm-St-Sh-M), abandoned channel(Sh-Fr-Mh-Sh-Fr-Mh), overbank deposits(Sh-Fr-Mh-Fr-Mh) and flood plain(M-Mh-Mc-Mh-M), and forms a sequence of normal sequence deposits with a binary structure unobviously;the vertical sedimentary sequence of meandering river consists of three types of microfacies, including stranded sediment(Gm), side beaches(Sp-St-Sm-Sp-St-Sh) and flood land(M-Mh-Mc(Fr)-Mh-M), and constitutes a typical binary structure sedimentary sequence.