柴达木盆地西部斜坡区下干柴沟组下段高精度层序地层及砂体构型分析

精细刻画砂岩储层内部结构,可为柴达木盆地西部(柴西)斜坡区古近系下干柴沟组下段岩性油气藏勘探开发指明方向。以高精度层序地层学、砂体构型分析理论和方法为指导,以3D地震、钻/测井、岩心资料为基础,井震联合标定,综合分析了柴西斜坡区下干柴沟组下段层序地层发育规律与层序格架内砂体结构特点。结果表明,柴西斜坡区下干柴沟组下段整体为一个区域性湖侵序列,内部可划分为2个三级层序,4个四级层序(EMSQ1～4)。下干柴沟组下段沉积期,由早至晚,A/S(可容纳空间与沉积物供给比值)持续增大,四级层序结构由向上变深的半旋回型逐渐转变为向上变浅为主、向上变深为次的非对称型。沉积相则由辫状河三角洲平原演变为前缘、滨浅湖亚相。研究区主要发育分流河道、水下分流河道、滩坝3类储层砂体,砂体叠加样式和分布规律与四级层序结构相关。三角洲平原分流河道砂体主要发育于EMSQ1中,砂体相互切割和叠置程度高,连通性好;三角洲前缘分流河道砂体主要发育于EMSQ2～3中,多呈叠加-孤立型,连通性差;滨浅湖滩坝砂体主要发育EMSQ4中,砂体呈孤立状,层内连通性佳,层间连通性差。指出现阶段岩性油气藏勘探应重点围绕EMSQ4层序内发育的滨浅湖滩坝砂体展开。

High-precision sequence stratigraphy and sandbody architecture of the Lower Xiaganchaigou Formation in the slope area of Western Qaidam Basin

Characterizing the internal architecture of the reservoir of the Lower Xiaganchaigou Formation in the slope area of Western Qaidam Basin can provide some reference for the exploration and development of lithologic reservoirs in this area. Guided by high-precision sequence stratigraphy, sandbody architecture theory and method, the authors studied the characteristics of sequence stratigraphy and sandbody architecture in the Lower Xiaganchaigou Formation on the basis of well-seismic combined calibration and comprehensive analysis of seismic, well logging, drilling and core data. The results show that the Lower Xiaganchaigou Formation in the slope area of Western Qaidam Basin is a large regional lacustrine invasion sequence, which can be divided into 2 third-order sequences and 4 fourth-order sequences (EMSQ1~4). From early to late, A/S (the ratio of accommodating space and sediment supply) continues to increase, whereas the sequence structure of the fourth sequence gradually changes from deepening-upward semi-cycle to non-symmetric deepening-upward full-cycle. Meanwhile, the sedimentary facies changes from braided river delta plain to delta front, and finally changes to shore-shallow lacustrine facies. The stacking patterns and distribution regularity of different types of sandbodies are closely related to the fourth-order sequence structure. The distributary channel sandbodies in delta plain facies are mainly developed in EMSQ1, and cut each other and overlap vertically so that their connectivity is very good. The subaqueous distributary channel sandbodies in delta front are mostly developed in EMSQ2~3, and are isolated or overlapped occasionally so that their connectivity is not very good. The beach-bar sandbodies of the shoreshallow lacustrine facies are chiefly developed in EMSQ4, and are isolated from each other so that their connectivity is bad. It is concluded that the further exploration of lithologic reservoirs should focus on shore-shallow lacustrine beach-bar sandbodies in EMSQ4 sequence.