桂中坳陷北部下石炭统鹿寨组页岩岩相类型及储层特征

Lithofacies types and reservoir characteristics of shale of Lower Carboniferous Luzhai Formation in northern Guizhong Depression

桂中坳陷北部下石炭统鹿寨组富有机质泥页岩是页岩气勘探的主力层位。文中运用扫描电镜、X射线衍射、氩离子抛光、氮气吸附实验等测试手段以及总有机碳质量分数分析,研究了该层位暗色泥页岩的的主要岩相类型、储层物性与孔隙发育特征,并指出了优势页岩岩相。结果表明:1)运用总有机碳质量分数与硅质矿物(石英+长石)、碳酸盐矿物(方解石+长石)、黏土矿物四端元法,可识别出硅质页岩、钙质页岩、黏土质页岩及混合质页岩等4大类13小类岩相类型,以富有机质硅质页岩、富有机质黏土质/硅质页岩、富有机质黏土/硅混合质页岩和中有机质黏土/硅混合质页岩为主。2)鹿寨组硅质页岩储集空间以有机质孔、矿物溶蚀孔为主,混合质页岩以矿物溶蚀孔、与黏土矿物质量分数相关的孔隙为主。页岩孔径分布表现为多峰特征,以微孔、介孔为主,宏孔相对欠发育。孔隙度在1.20%~6.21%,平均值为2.29%,渗透率介于0.001×10^(-3)~0.023×10^(-3)μm^(2),平均值为0.004×10^(-3)μm^(2)。页岩吸附气量普遍高于1.5 m3/t,分布特征与孔隙度具有高度一致性。3)富有机质硅质页岩的总有机碳质量分数、石英等矿物质量分数、孔隙度及吸附气量均较高,可作为优势页岩岩相,其次为富有机质黏土/硅混合质页岩。优势页岩岩相主要发育于深水盆地或深水陆棚沉积,因此宜山断凹构造单元带内的深水台盆沉积可作为主要有利勘探目标,其次为柳城斜坡与环江浅凹的深水陆棚沉积。

The organic-rich shale of Lower Carboniferous Luzhai Formation in northern Guizhong Depression is the main target layer for shale gas exploration.By means of test methods such as scanning electron microscopy,X-ray diffraction,argon ion polishing,nitrogen adsorption experiment and total organic carbon mass fraction analysis,the main lithofacies types,reservoir physical properties and pore development characteristics of the dark shale in this layer are studied,and the dominant shale lithofacies are pointed out.The results show that:(1)By using the method of total organic carbon mass fraction and four terminal elements of siliceous minerals(quartz+feldspar),carbonate minerals(calcite+feldspar)and clay minerals,13 small types of four categories including siliceous shale,calcareous shale,clay shale and mixed shale can be identified.It is mainly composed of organic-rich siliceous shales,organic-rich clay/siliceous shales,organic-rich clay/siliceous mixed shales and medium organic-rich clay/siliceous mixed shales.(2)The reservoir space of Luzhai Formation siliceous shale is dominated by organic matter pores and mineral dissolution pores,and mixed shales are dominated by mineral dissolution pores and pores related to the mass fraction of clay minerals.The pore diameter distribution of shale is characterized by multi-peaks,mainly developing micropores and mesopores,while macropores are relatively underdeveloped,the porosity ranges from 1.20%to 6.21%,and the average value is 2.29%.The permeability ranges from 0.001×10^(-3)to 0.023×10^(-3)μm^(2),and the average value is 0.004×10^(-3)μm^(2).The adsorption capacity of shale is generally higher than 1.5 m3/t,and its distribution characteristics are highly consistent with its porosity.(3)The total organic carbon mass fraction,quartz and other minerals mass fraction,porosity and adsorption capacity of organic-rich siliceous shale are high,which can be used as the dominant shale lithofacies,followed by organic-rich clay/silicon mixed shale.The dominant shale lithofacies are mainly developed in deep-water basin or deep-water shelf sediment,so the deep-water platform basin sediment in the Yishan fault depression tectonic unit belt can be the main favorable exploration target,followed by the deep-water shelf sediment in Liucheng slope and Huanjiang shallow depression.