四川盆地上奥陶统五峰组—下志留统龙马溪组页岩关键矿物成岩演化及其控储作用

Diagenetic evolution of key minerals and its controls on reservoir quality of Upper Ordovician Wufeng-Lower Silurian Longmaxi shale of Sichuan Basin

利用岩心—薄片—扫描电镜观察、X-射线衍射分析、碳酸盐岩碳氧同位素和能谱分析等手段,对四川盆地上奥陶统五峰组—下志留统龙马溪组页岩中的石英、长石、黄铁矿、碳酸盐类和黏土矿物进行了有效识别与划分,分析了矿物成岩演化序列及其对储层发育的影响。结果表明,良好的物质基础和独特的成岩改造是优质页岩储层形成的关键:(1)莓状/自形黄铁矿、生物石英和微生物白云石主要形成于同生—早成岩阶段早期,对页岩原始孔隙的保持具有破坏性和建设性双重作用,其建设性支撑格架作用对优质页岩的形成起决定性作用,此类矿物与陆源碎屑构成的刚性支撑格架有利于原始孔隙的保持与后期的压裂改造。(2)生烃—成岩协同演化促进储集空间的发育,中成岩早期有机酸的产生和消耗、不稳定矿物(长石和碳酸盐矿物)溶蚀/蚀变、黏土矿物转化和干酪根生油具有同步性,为生油期液态烃的充注与滞留提供了有利空间;中成岩晚期—晚成岩阶段,干酪根和滞留烃裂解生气、成孔和增压促进了有机孔与微裂缝的发育,利于页岩气的富集与高产。

Based on core,thin section,scanning electron microscopy observations,X-ray diffraction analysis,as well as carbon and oxygen isotopes and energy spectrum analysis of carbonate rocks,the quartz,feldspar,pyrite,carbonate and clay minerals in shale of the Upper Ordovician Wufeng-Lower Silurian Longmaxi formations of Sichuan Basin were effectively characterised and classified,and the influences of their diagenetic evolution sequence on the development of shale reservoir were discussed.The results showed that good material basis and unique diagenetic sequence were the key factors for the formation of high-quality shale reservoirs.(1)Framboidal/euhedral pyrite,bio-quartz and microbial dolomite were mainly formed from the syngenetic stage to the A-substage of early diagenetic stage.They were both destructive and constructive for maintaining the original pores in shale,and the constructive supporting framework of which was critical for the formation of high-quality shale reservoir.The rigid framework formed by these early-formed minerals and terrigenous debris facilitated the maintenance of original pores and the reservoir stimulation of shale gas exploitation.(2)The co-evolution of hydrocarbon generation and diagenesis promoted the development of reservoir spaces.In the A-substage of middle diagenetic stage,the production and consumption of organic acids,the dissolution/alteration of unstable minerals(feldspar and carbonate minerals),clay mineral conversion and oil generation from kerogen were synchronic,which provided favorable space for the charging and retention of liquid hydrocarbons during the oil generation period.From the B-substage of middle diagenetic stage to the late diagenetic stage,the shale gas/organic pore generation and pressure increase of kerogen and retained hydrocarbon cracking promoted the development of organic pores and micro-fractures,which was conducive to the enrichment and high production of shale gas.