川东北下侏罗统大安寨段陆相页岩方解石成因

Genesis of Calcite in Nonmarine Shale of the Lower Jurassic Da’anzhai Member,Northeastern Sichuan

为分析陆相页岩中方解石的成因及其对储层的影响,以四川盆地下侏罗统大安寨段的介壳泥岩与灰岩夹层为主要研究对象,利用X-射线衍射、普通薄片、岩芯观察、阴极发光及电子探针测试手段,识别出文石转化方解石、胶结物方解石和重结晶作用形成的方解石。在同生期和成岩早期,生物介壳中的文石转化为泥晶无铁方解石,阴极发光为橙黄色。成岩早期,生物介壳边缘可形成纤维状第一世代无铁方解石胶结物。在成岩晚期,岩石孔隙中形成颗粒状铁方解石胶结物,阴极发光为暗色。同时,部分介壳中的泥晶方解石新生变形为斑块状细晶方解石或柱状方解石。根据方解石的存在形式可知,方解石在成岩演化过程中,经历了转化、压实、溶解、胶结和重结晶作用。其中,溶解作用改善储层的物性。压实、胶结和重结晶作用破坏储集空间。统计裂缝密度发现,元坝地区大安寨段层理缝最为发育,涪陵地区大安寨段以层理缝和溶蚀缝为主。结果表明,方解石介壳有利于大安寨段内岩石层理缝和溶蚀缝的产生。

Shell mudstone interbedded with limestone from the Da′anzhai member of Sichuan Basin was studied in this investigation of the genesis of calcite in nonmarine shale and its influence on reservoirs.X-ray diffraction(XRD)analysis and observation of thin sections and cores,cathodoluminescence,and electron probe microanalysis(EPMA)test methods were used to identify the calcites transformed by aragonite,and those formed by cementation and recrystallization.In the syngenetic and early diagenetic period,aragonite in the shells was transformed into mi-critic non-ferroan calcite,evident as orange-yellow cathode luminescence.In the early diagenetic period,the first generation of non-ferroan calcite cement was formed at the edges of the shells.In late diagenesis,granular ferroan cal-cite cement was formed in the intergranular pores,and the cathode luminescence appears dark.Some micritic calcite in the shells was transformed into pitted fine crystalline or columnar calcite.Depending on its form,the calcite under-went transformation,compaction,dissolution,cementation and recrystallization in the diagenetic evolution process.Dissolution improves the physical properties of the reservoir,but compaction,cementation and recrystallization pro-cesses destroy the reservoir space.Measures of fracture density indicate that the most highly developed lamellation fractures are found in the Da′anzhai member in the Yuanba area,whereas mainly lamellation fractures and dissolu-tion fractures are found in the Da′anzhai member of the Fuling area.The study shows that the presence of calcite shells has benefited the formation of the latter fracture types.