湖泊深水重力流沉积露头精细解剖——以鄂尔多斯盆地瑶曲剖面长7油层组为例

Outcrop-Based Analysis of a Deep-Water Gravity Flow Sediments in Lake:A Case Study from the Chang 7 of Yaoqu Section,Ordos Basin

沉积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不可替代的作用。本文以鄂尔多斯盆地南部瑶曲剖面长7段为例,采用地质学、野外露头沉积学方法,详细剖析了湖泊深水重力流沉积深水岩相的类型、特征、垂向组合及沉积环境。研究表明,瑶曲剖面长7段发育块状细砂岩相、鲍马序列粉砂岩相、水平纹理泥岩相、块状泥岩相、水平层理页岩相、凝灰岩相6种岩相类型,碎屑流沉积、浊流沉积、滑塌沉积、深水原地沉积、火山碎屑沉积5类沉积单元,在垂向上5类沉积单元构成了A、B、C、D 4类垂向分布形式。在此基础上,结合区域地质特征,研究认为长7段发育湖泊、水道型重力流沉积两种沉积体系。湖泊相可进一步确定为半深湖—深湖亚相,深水原地沉积微相,以D类岩相组合最为发育;水道型重力流沉积体系进一步划分为近源水道、远源水道、堤岸、前端朵体4个亚相,包含砂质碎屑流沉积、浊流沉积、滑塌沉积3个微相,近源水道+堤岸形成A类、C类、B类岩相组合,远源水道+堤岸形成B类、C类岩相组合,前端朵叶发育C类岩相组合。剖面相分析表明,研究区优质的烃源岩与重力流砂体可构成较好的"下生上储"的致密岩性油气藏。

Outcrop-based sedimentary section plays an irreplaceable role in detailed analysis for internal architecture of sedimentary bodies and building accurate underground geological model. Taking the Chang 7 oil layer of Yaoqu Section developed in the south of Ordos Basin as an example, we discusses the type, characteristics, vertical sequences and sedimentary environment of deep-water gravity flow deposits of deep-water lithofacies employing geology, field outcrops and sedimentology. It is suggested by this study that six lithofacies including massive fine silt facies, Bouma Sequence siltstone facies, horizontal lamination bedding mudstone facies, massive mudstone facies and tuff facies, and five kinds of sedimentary units including debris flow deposit, turbidite deposit, slump deposit, deep-water original deposit and tephra deposit have occurred in Chang 7. Five kinds of sedimentary units constitute the distribution of A, B, C and D in vertical direction. In combination with the regional geological features, our study suggests that the sedimentary system developed in Chang 7 include mainly lake and channel gravity flow deposit. Lacustrine facies can be further divided into shore-shallow lake facies, deep-water in-situ sedimentary microfacies, with D type as the most developed lithofacies association. Channel gravity flow deposit can be further divided into four subphases, such as near source channel, far source channel, embankment and front lobe bodies, including three micro phases such as sandy debris flow deposit, turbidite deposit and slump deposit. Near source channel ＋ embankment formed lithofacies associations of class A, class C and class B, and far source channel ＋ embankment formed lithofacies associations of class B and class C, and front lobe developed into class C lithofacies association. The profile phase analysis shows that the top- quality hydrocarbon rocks and gravity flow sandbody in the study area could form favorable tight lithological oil and gas reservoir with ＂lower-generation and upper storage＂.