吐哈盆地胜北洼陷中侏罗统致密砂岩储层孔隙发育特征及控制因素

Pore characteristics and its controlling factors in the Middle Jurassic tight sandstone reservoirs of the Shengbei Sag,Turpan-Hami Basin

吐哈盆地中侏罗统致密砂岩储层是非常规致密油气勘探开发的主要目的层。以胜北洼陷8口井中侏罗统致密砂岩样品为主要研究对象,从储层岩石学特征、成岩作用、物性、孔隙结构等方面系统刻画了研究区中侏罗统致密砂岩储层特征,探讨了影响储层孔隙发育的主要因素。研究结果表明胜北洼陷中侏罗统低孔-特低渗致密砂岩储层以长石岩屑砂岩为主,岩屑砂岩次之;该储层遭受了强烈的压实作用,矿物胶结、交代和溶蚀现象明显,孔隙类型以次生长石溶蚀孔为主,同时发育矿物残余粒间孔、石英粒内溶蚀孔、黏土矿物层间孔和微裂缝等;储层内5~50 nm孔喉最为发育,然而储层物性则主要受50 nm~1μm和100~800μm孔喉发育程度控制,主要体现在两者孔喉体积与孔隙度和渗透率呈良好的正相关关系。储层物性与石英和长石含量呈正相关关系,与黏土矿物和碳酸盐矿物含量呈负相关关系,其原因一方面是该储层遭受了强烈压实,石英颗粒破裂导致其内微裂缝较发育,刚性石英含量增加有利于保存部分原始粒间孔,其内溶蚀孔也较发育,含有机酸的烃类流体运移至中侏罗统地层内促使长石发生了大规模溶蚀,形成大量次生长石溶蚀孔,孔径50 nm~1μm和100~800μm孔喉相对发育,储层物性因此得以改善;另一方面黏土矿物和碳酸盐矿物多以胶结物形式充填在原始粒间孔和次生微裂缝内,方解石交代长石降低了长石溶蚀的增孔效应,不利于孔径50 nm~1μm和100~800μm孔喉发育和储层物性改善。因此,研究区储层孔隙发育与早期原始沉积环境和后期成岩作用关系密切,压实作用、长石溶蚀和自生矿物胶结对储层孔隙发育及物性具有关键控制效应。该研究对吐哈盆地胜北洼陷中侏罗统致密砂岩油气有利勘探区分布预测具有指导意义。

The Middle Jurassic sandstone reservoirs of the Turpan-Hami Basin are the main targets for unconventional tight oil and gas exploration and development.To better understanding the main controlling factors of pore characteristics in the Middle Jurassic tight sandstones of the Shengbei Sag,Turpan-Hami Basin,a comprehensive investigation referring to lithology,diagenesis,physical properties,and pore structure was conducted on samples obtained from 8 wells.The results indicate that the Middle Jurassic tight sandstone reservoir characterized by low porosity and ultralow permeability primarily contains feldspar lithic sandstones,followed by lithic sandstones.These sandstone reservoirs suffered strong compaction and complicated mineral cementation,replacement,and dissolution.These sandstone reservoirs are dominated by the secondary pores generated by feldspar dissolution,with some residual interparticle pores,quartz dissolution pores,clay mineral interlayer pores,and microfractures.Pore throat of 5-50 nm most widely appears in these reservoirs,while the porosity and permeability of these reservoirs mainly depend on the pore throats of 50 nm-1μm and 100-800μm,as indicated by the good positive correlations between pore throats and volumes.Both porosity and permeability correlate positively with the quartz and feldspar contents but negatively with the clay and carbonate contents.These correlations were supposed to be caused by two factors:①strong compaction led to the loss of most interparticle pores but kept the residual interparticle pores associated with rigid quartz,and caused the occurrence of microfractures and dissolution pores within the quartz;the migration of hydrocarbon fluids containing organic acids into the Middle Jurassic reservoirs resulted in the significant dissolution of feldspar and generation of secondary dissolution pores.This process promoted theoccurrence of pore throats of 50 nm-1μm and 100-800μm and improved the porosity and permeability.②The primary interparticle pores and secondary microfractures were filled with authigenic clay or carbonate cement;the replacement of feldspar by calcite disturbed the positive effects of feldspar dissolution on the porosity and permeability,which reduced the occurrence of 50 nm-1μm and 100-800μm pore throats and the physical properties of these reservoirs.Therefore,the occurrence of pores was closely related to the early sedimentary environments and the later diagenesis after deposition.More importantly,mechanical compaction,feldspar dissolution,and authigenic mineral cementation played crucial roles in regulating the occurrence of pores and the physical properties of these sandstone reservoirs in the study area.This study should be helpful in predicting the favorable exploration areas of tight oil and gas in the Middle Jurassic sandstone reservoirs in the Shengbei Sag,Turpan-Hami Basin.