川西南中二叠统中粗晶白云石流体来源分析  

Origins of Dolomitization Fluids within Middle Permian Coarse Dolomite,SW Sichuan Basin

作　　者：郑浩夫 袁璐璐 刘波[1,2] 张学丰[1,2] 沈瑛楚 王远翀 ZHENG HaoFu;YUAN LuLu;LIU Bo;ZHANG XueFeng;SHEN YingChu;WANG YuanChong(School of Earth and Space Sciences,Peking University,Beijing 100871,China;Institute of Oil&Gas,Peking University,Beijing 100871,China;Chinese Academy of Geological Sciences,Beijing 100037,China;SinoProbe Center,Chinese Academy of Geological Sciences and China Geological Survey,Beijing 100037,China)

机构地区：[1]北京大学地球与空间科学学院,北京100871 [2]北京大学石油与天然气中心,北京100871 [3]中国地质科学院,北京100037 [4]中国地质调查局中国地质科学院地球深部探测中心,北京100037

出　　处：《沉积学报》2020年第3期589-597,共9页Acta Sedimentologica Sinica

基　　金：国家自然科学基金项目(41672123);国家科技重大专项(2017ZX05005-003-005)。

摘　　要：四川盆地西南地区中二叠统地层在埋藏过程中发生了较高程度的白云岩化。通过野外剖面观察和详细的薄片岩石学研究,在中二叠统白云岩储层中识别出了四种类型的白云石(包括三种基质交代白云石和一种白云石胶结物):1)粉晶白云石,宏观上主要呈层状发育,晶粒小于50μm,平直镜面半自形晶—非平直晶面它形晶;2)细晶白云石,晶粒大小为50~250μm,平直晶面半自形晶—自形晶;3)中粗晶白云石,宏观上可见溶蚀孔洞和裂缝发育,其中充填白色的白云石胶结物、方解石胶结物等,晶粒大小为250μm^2 mm,非平直晶面它形晶;4)白云石胶结物,以胶结物的形式在裂缝和溶蚀孔洞中发育,晶粒大小变化较大,具有明显的波状消光。利用不同矿物之间的接触和切割关系,结合阴极发光和扫描电镜等手段,确定了几种白云石和相关成岩矿物的形成时序,确立四川盆地西南地区中二叠统白云岩的成岩演化序列。即从成岩早期到晚期,依次形成(或发生成岩作用)了粉晶白云石、早期溶蚀作用、细晶白云石、中粗晶白云石、水力压裂缝、白云石胶结物、石英、方解石脉、缝合线、晚期溶蚀和沥青充填。通过地球化学和包裹体分析,发现中粗晶白云石和白云石胶结物具有相似的地球化学特征,即明显偏负的氧同位素、大于同期海水的Sr同位素,成岩流体具有较高的温度和盐度,表明其成岩流体具有典型的热液性质。原始灰岩和早期白云岩经热液改造,重结晶为中粗晶白云石,并在裂缝和溶蚀孔洞中沉淀鞍形白云石胶结物。The Middle Permian Qixia and Maokou Formations in the southwestern Sichuan Basin were pervasively do-lomitized during their diagenetic history.Four phases of dolomites,including three replacive dolomites(Rd1,Rd2,and Rd3)and one dolomite cement(Cd)were distinguished based on petrographic study.Rd1 dolomite occurs as very fine(<50μm),planar-s to nonplanar crystals;Rd2 dolomite shows planar-e to planar-s crystal shapes with fine crystal sizes(50-250μm)and is characterized by a fogged center with a clear margin;Rd3 dolomite occurs as medium to coarse(250μm-2 mm),nonplanar crystals;Cd dolomite is characterized by saddle crystals filling disso-lution pores and/or fractures,with a translucent white color in the hand samples and strong sweeping extinction under cross polarized light.In this study,the diagenetic sequence of the four types of dolomite and related minerals were identified based on their petrographic characteristics,distributions,and crosscutting relationships with other diage-netic events.From start to finish,micritic dolomite,early phase dissolution,fine-grained dolomite,medium to coarse dolomite,hydrofracturing fractures,dolomite cements,quartz cements,calcite cement in fractures,burial stylolite,late phase dissolution,and bitumen formed in sequence.Through geochemical and inclusion analysis,we found that the medium-coarse dolomite and dolomite cement have similar geochemical characteristics,i.e.,the oxy-gen isotope is obviously negative,and the Sr isotope is larger than the seawater in the same period.The diagenetic flu-id has a higher temperature and salinity,indicating that it has typical hydrothermal properties.The original limestone and early dolomite were hydrothermally reformed and recrystallized into medium-macrocrystalline dolomite,and the saddle-shaped dolomite cement precipitated in cracks and dissolved pores.

关 键 词：川西南地区 白云岩 岩石学特征 成岩演化序列 流体来源 

分 类 号：P618.13[天文地球—矿床学]