大型辫状河席状复合河道岩性圈闭识别描述方法及应用

Study and application of identification and description methods for lithologic traps in large braided river sheet composite channels

鄂尔多斯盆地二叠系致密砂岩气藏以近源、致密、大面积含气、沿主河道分布但非均质性强为特点,受砂体内部物性变化的控制,气水分异不明显,含气边界模糊不清。因此,这种类型气藏的圈闭识别描述并非通常意义上油气藏的单个圈闭识别描述,而必须以有效储集单砂体的空间叠置形成的复合河道砂体的展布预测为主要内容,描述砂体空间结构关系以确定最大储量控制单元"甜点区"。以盆地北部杭锦旗地区独贵加汗下二叠统下石盒子组一段(盒1)气藏为例,研究大型辫状河席状复合河道岩性圈闭的特征及识别描述方法。以层序格架为约束,地质模式为基础,井—震联合进行沉积相分析,实现辫状河河道砂岩有效储集体及其边界分布的有效预测与刻画;建立了此类圈闭评价六步法识别描述技术流程,能够实现利用相对低分辨率地震资料对辫状河席状复合河道中有效储集体进行有效预测。随着技术系列不断应用与完善,在独贵加汗盒1段岩性圈闭探明天然气储量已超千亿立方米。

The gas reservoirs in the Permian tight sandstones in the Ordos Basin are characterized by near source,tightness,large-scale area of gas reservoir,distribution along the main channel,and strong heterogeneity.Influenced by the changes in the internal physical properties of sand bodies,the difference between gas and water is not obvious,and the gas boundary is also unclear.Therefore,the trap identification and the description of this type of gas reservoir is not like those for a single trap in the ubiquitous cases.It should be focused on the distribution of the composite channel sand bodies formed by the spatial superposition of effective single sand body,so as to determine the“sweet spots”of the maximum reserve unit.A case study was made for the first member of Xiashihezi Formation of Lower Permian in the Duguijiahan trap of Hangjinqi area,northern part of Ordos Basin.The characteristics as well as identification and description methods for lithologic traps in sheet-shaped composite channels of large-scale braided rivers were discussed.Constrained by the sequence framework and based on the geological model,a well-seismic joint analysis of sedimentary facies was employed here to realize the effective prediction and description of effective sandstone reservoirs and their boundary distributions in braided river channels.A six-step evaluation of this type of trap was introduced,which could predict the effective reservoirs in sheet-shaped composite channels of braided rivers by using relatively low-resolution seismic data.With the continuous application and improvement of the technical series,the proven natural gas reserves in lithological traps of the first member of Xiashihezi Formation in the Duguijiahan trap have exceeded 100 billion cubic meters.