多尺度河流相致密砂岩储集层表征及控制因素分析:以苏里格气田下石盒子组8段为例

Multiscale description and controlling factors analyses of fluvial tight reservoir:a case study on the 8th Member of Lower Shihezi Formation in Sulige gas field,Ordos Basin

以鄂尔多斯盆地苏里格气田苏6区块下石盒子组8段(盒8段)作为重点研究对象,通过野外地质剖面观察、岩石样品显微组分鉴定及定量、孔隙结构分类及定量表征,从砂体结构特征、砂岩类型、孔隙类型、微观孔隙结构特征等方面,多手段、多尺度全面剖析河流相砂岩,探讨河流相致密砂岩有效储集层的控制因素。结果表明:心滩(或边滩)粗砂岩与河道充填沉积细砂岩之间在岩性、物性、孔隙结构等方面存在明显差异。中粗砂岩孔隙类型以溶蚀孔隙为主,呈粗大的、连通成片的管状孔喉,有效孔喉主要为半径大的孔喉;细砂岩孔隙主要为黏土矿物晶间孔隙,在空间分布上主要为小球状或短管状,呈孤立状零散分布,有效孔喉大都集中在小孔喉部分。中粗粒岩屑石英砂岩为强硅质胶结、较强机械压实致密化模式,中粗粒岩屑砂岩为强机械压实致密化模式,细粒(长石)岩屑砂岩为强压实、较强硅质胶结致密化模式,砂岩致密化模式的差异,导致现今储集层中中粗砂岩的粗大孔喉和有效孔喉的比例、流体可动用能力、物性大小及相关程度明显高于细砂岩。

In this study,we carries out a comprehensive work of outcrop section observation,microscopic petrology investigation and quantitatively analysis of microscopic pore structure to reveal the fluvial reservoir characteristics of the 8th Member of the Lower Shihezi Formation of Su6 Area,Sulige Gas Field.Sand body structure characteristics,sandstone types,pore types,microscopic pore structure characteristics are detailed analyzed to indicate the controlling factors of effective fluvial tight sand reservoir.Our work demonstrates that the sand body of channel bar and point bar are the flooding event deposition and are medium-coarse grained lithic quartzarenite and litharenite.On the contrary,the channel fill microfacies sand body is the normal river flow period deposition,the associated sandstone is characterized by fine grained(feldspathic)litharenite and lower structure maturity.The medium-coarse grained sandstones have a large proportion of dissolved and interconnected pores with larger radius effective pore-throat.The pore types of fine-grained sandstones are mainly clay minerals intercrystalline pores,and the pore-throat radius is much smaller.Densification modes of fluvial reservoir can be divided into three types,such as densification of medium-coarse grained lithic quartzarenite by strong siliceous cementation and medium to strong compaction,densification of medium-coarse grained litharenite by strong compaction,densification of fine grained(feldspathic)litharenite by strong compaction and medium to strong siliceous cementation.The two types of medium-coarse grained sandstones are densification after hydrocarbon charge,which experience injection of organic acid and make dissolved pore to be the major storage space.Fine grained(feldspathic)litharenite is densification before hydrocarbon charge,which experiences weak dissolution.Differences of densification modes result in variable pore structure characteristics and reservoir physical properties.