四川盆地东南缘二叠系龙潭组泥页岩成岩作用特征及其对孔隙演化的影响

Diagenesis of shale and its influence on pore evolution in the Permian Longtan Formation,southeastern Sichuan Basin

通过薄片鉴定、场发射扫描电镜观察及X—射线衍射分析等方法,对四川盆地东南缘二叠系龙潭组泥页岩成岩作用特征及孔隙特征进行研究,并探讨成岩作用对储层孔隙演化的影响。研究结果表明:①龙潭组泥页岩主要成岩类型包括压实作用、胶结作用、溶蚀作用、有机质热成熟作用和黏土矿物转化作用。②储层孔隙以粒间孔为主,主要是黏土矿物之间的孔隙,少量晶间孔和碎屑颗粒之间的格架孔和粒缘孔(缝),部分有机质孔和溶蚀孔。③在成岩早期,压实作用是储层孔隙减少的主要原因,胶结作用使得岩石孔隙进一步被填充,却在一定程度上增强了岩石的抗压实作用;随埋深增加,压实作用强度开始减弱,有机质热演化对储层孔隙改造有限,岩石中的溶蚀作用较弱,使得储层孔隙进一步降低;之后,随埋深增加,黏土矿物的大量转化使岩石中孔隙减少,岩石结构更加致密,同时石英及碳酸盐胶结物的形成,使得储层孔隙进一步降低,但黏土矿物作为龙潭组泥页岩的主要矿物类型,黏土矿物间的微孔依然是龙潭组中主要的孔隙类型。

Through thin-section identification,field-emission SEM observation and X-ray diffraction analysis,both diagenesis and pore characteristics were studied for the shale in the Permian Longtan Formation,southeastern Sichuan Basin.Moreover,the diagenetic influence on pore evolution was discussed.Results show that(1)there are five kinds of diagenesis in the shale containing compaction,cementation,dissolution,thermal maturation of organic matter,and transformation of clay minerals;(2)the reservoir space is dominated by intergranular pores(especially among clay minerals),with a few intercrystalline and framework pores or edge pores(fractures)among clastic particles,and some organic-hosted and dissolution pores;and(3)in the early diagenesis,compaction was the main causing decease in pores,cementation made these pores be further filled,but enhanced the compaction resistance to a certain extent.With the increase of burial depth,the compaction intensity began to weaken,the thermal evolution of organic matter might restrict the pore reformation,and weak dissolution further reduced the porosity.After that,with the further increase of burial depth,the massive transformation of clay minerals reduced the pores and made the rock structure more compact.At the same time,the formation of quartz and carbonate cements lowered the pores.However,clay minerals are still the essential in the shale and micropores among them function as the major pore type in Longtan Formation.