四川盆地东部茅口组白云岩成因:来自岩石学、矿物学和地球化学的证据  

Origins of dolostones of the Maokou Formation in eastern Sichuan Basin:evidence from lithology,mineralogy,and geochemistry

作　　者：王良军[1,2] 李红 曾韬[1] 柳益群 潘磊[1] 李让彬[1] 李文厚[3] 张冬冬[3] 焦鑫 杨康[3] 董杨坤 WANG Liangjun;LI Hong;ZENG Tao;LIU Yiqun;PAN Lei;LI Rangbin;LI Wenhou;ZHANG Dongdong;JIAO Xin;YANG Kang;DONG Yangkun(Sinopec Exploration Company,Chengdu 610041,China;Sinopec Henan Oilfield,Henan Nanyang 473132,China;State Key Laboratory of Continental Dynamics,Northwest University,Xi'an 710069,China)

机构地区：[1]中国石化勘探分公司,四川成都610041 [2]中国石化河南油田分公司,河南南阳473132 [3]大陆动力学国家重点实验室,西北大学,陕西西安710069

出　　处：《古地理学报》2022年第5期989-1016,共28页Journal of Palaeogeography：Chinese Edition

基　　金：中国石化重大科技项目(编号:P16082);国家自然科学基金项目(编号:41272115,41572086)联合资助。

摘　　要：四川盆地中二叠统茅口组白云岩中蕴藏丰富的天然气资源,但是这套白云岩的成因多年来一直存在争议。盆地东部茅口组白云岩储集层多发育在茅三段中下部,纵向上厚度介于3.0~46.8 m之间,平面上沿北西—南东向基底断裂呈带状展布。野外和钻井岩心观察发现,白云岩内发育交织状张性裂缝,将岩石切割呈角砾状,裂缝多被粗晶方解石、白云石充填或半充填。根据围岩和裂缝中白云石晶体形貌和产出状态,在显微镜下识别出4类不同组构的白云石:第1类为灰泥基质中“星散状”自形粉晶—细晶白云石;第2类为半自形面状组构细晶白云石,呈他形—半自形镶嵌状;第3类为具“雾心亮边”的自形晶面状组构细晶—粗晶白云石,具糖粒状结构,晶间孔隙发育;第4类为脉体充填的粗晶鞍状双晶白云石。背散射照片显示前3类白云石的“雾心”部分晶面混浊、表面分布方解石残斑及微孔,而第3类白云石的“亮边”部分与第4类鞍状粗晶白云石脉则晶体明亮、洁净、致密,属新生矿物。上述4类白云石的成分均具有富Ca和贫Fe、Mn、Sr的特点,MgO含量变化较大,其中前两类白云石MgO含量总体低于白云石标准计量,而后两类白云石的MgO含量则接近或略高于标准计量。岩石学、矿物学和地球化学特征表明:茅口组白云岩主要经历了2期白云化作用,初次白云化作用形成第1类、第2类白云石及“雾心”白云石,二次白云化作用形成“亮边”白云石和粗晶白云石脉体;2期白云化作用均发生得很早,可能始于同沉积期至浅埋藏期。白云石原位微量元素和原位锶同位素分析表明,白云化作用流体具有混源的特点,流体可能由富Ca地下水以及与火山活动有关的热液混合而成。推测中二叠世末期峨眉山地幔柱的喷发引起四川盆地内基底断裂再次活动,造成茅口组内张性构造裂缝发育,这为白云化流体混�Dolostones in Middle Permian Maokou Formation in the Sichuan Basin contain abundantnatural gas resources but their origins are controversial for years.The dolostones are present in the lower-tomiddle-parts of the Member 3 of Maokou Formation,with thickness ranging from 3.0 to 46.8 m.The la-teral distribution of Maokou dolostone extends as a NW-SE belt along a basement fault system.Field anddrilling core observations show that the dolostones contain numerous tension fractures,fully-or partly-filledby coarse dolomite and calcite as veins,sometimes presenting a breccia structure.Four textural consti-tuents are identified under the microscope:(Ⅰ)Silt-to-fine crystalline scattered euhedral dolomite floatingin lime mud matrix;(Ⅱ)Fine crystalline planar-s dolomite(mostly subhedral crystals packed as a mosaictexture);(Ⅲ)Fine-to-coarse crystalline planar-e dolomite with“cloudy center”and“clear rim”(su-crose texture with high intergranular porosity);(Ⅳ)Coarse saddle dolomite in veins.BSE(Backscat-tered Electron)images confirmed that there are lots of micropores and tiny calcite residuals on crystal sur-face of“cloudy center”dolomite,while typeⅢ“clear rim”dolomite and type IV dolomite in veins re-veal clean and smooth crystal face without micropores,indicating an origin of authigene during dolomitiza-tion.All the four types of dolomite show similar features of rich in calcium,poor in iron,manganese andstrontium,but their content of MgO varies differently.MgO(%)of type I and type II dolomite is alwayslower than that of stoichiometry dolomite,however,MgO(%)in typeⅢand type IV dolomite is closeto or even higher than the stoichiometry.Lithological,mineralogical and geochemical features indicate twoepisodes of dolomitization for the formation of Maokou dolostones.TypeⅠ,typeⅡand typeⅢ“cloudycenter”dolomite were formed during the first episode of dolomitization;and typeⅢ“clear rim”and typeⅣvein dolomite were authigenic during the second episode of dolomitization.Both episodes of dolomi

关 键 词：四川盆地 中二叠统 茅口组 白云岩 张性裂缝 热液白云化作用 

分 类 号：P575[天文地球—矿物学]