川东地区凉高山组湖泊三角洲砂泥互层背景下致密砂岩储层成因机制

Genetic mechanism of tight sandstone reservoir of lacustrine-delta sand-mud interlayer in Lianggaoshan Formation of Jurassic,eastern Sichuan Basin

川东地区凉高山组湖泊三角洲砂岩致密油气资源潜力巨大,明确储层成岩演化与成因机制是致密储层分布预测的关键。针对凉高山组砂岩储层成因机制认识不清的问题,通过铸体薄片、X射线衍射、扫描电镜、物性分析、核磁共振和高压压汞等分析手段系统研究分析了凉高山组砂岩的岩石学、储集空间、物性及成岩作用特征,综合分析储层成因机制。结果表明:①凉高山组砂岩主要为细粒岩屑砂岩,分选磨圆中等,储集空间以残余原生孔隙、粒间溶孔和粒内溶孔为主,孔隙度范围为0%~8%,渗透率范围主要为(0.001~0.01)×10^(-3)μm^(2),总体为低孔—低渗透致密储层;②破坏性成岩作用主要为机械压实作用,保持性成岩作用包括方解石胶结和自生绿泥石,建设性成岩作用主要为溶蚀作用;③高位体系域水下分支河道砂岩是优质储层发育的物质基础,机械压实作用是储层致密化主要原因,绿泥石包膜和早期孔隙方解石保护原生孔隙,长石和早期方解石溶蚀形成次生孔隙,烃源岩演化提供重要有机酸,水下分支河道砂体底部(砂体主体)储层物性优于砂体中上部(砂体边缘)。该认识为凉高山组优质储层分布预测提供了重要的理论支撑。

Lacustrine-delta sandstone of Lianggaoshan Formation has great potential of tight oil and gas resources in eastern Sichuan Basin.It is the key to determine diagenetic evolution and genetic mechanism of tight reservoir distribution prediction.However,genetic mechanism of Lianggaoshan Formation sandstone reservoir was still poorly understood in the eastern Sichuan Basin.Therefore,thin section,X-ray diffraction,scanning electron microscopy,physical property analysis,NMR porosity and high-pressure mercury injection were used to systematically study the petrology,reservoir space,porosity and permeability,and diagenesis.Moreover,genetic evolution mechanism of sandstone reservoir was also systematically analyzed.The main conclusions are as follows:(1)Lianggaoshan Formation sandstone mainly comprise fine-grained lithic sandstone and minor medium-grained and silty lithic sandstone with moderate sorted grains.The reservoir space mainly consists of intergranular dissolved pores,intragranular dissolved pores and primary pores.The porosity ranges from 0 to 8%,and the permeability ranges mainly from 0.001×10^(−3)μm^(2) to 0.01×10^(−3)μm^(2).(2)Diagenesis mainly includes mechanical compaction,calcite cementation,authigenic chlorite and dissolution.Calcite cements include early pore filling and late replacement of grains.Chlorite mainly occurs as grain coating.The feldspar and calcite cements were main subject of dissolution.(3)The underwater branching channel of high-level system was the mass basis of high-quality reservoir development.Mechanical compaction was the main cause of reservoir densification.The early chlorite coating can protect the pores and provide space for the subsequent precipitation of calcite cementation.The early pore-filling calcite decreased porosity by occupying the pore space.Feldspar and early calcite can produce secondary pore through dissolution.Source rock evolution can provide organic acid and for dissolution.The physical property of reservoirs at the bottom of the sandbody(the main body of sandbody)is better than those in the middle and upper part of the sandbody(the edge of the sandbody).This study can provide important theoretical support for the reservoir prediction.