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*

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,这为白云化流体混合及运移提供了通道。

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

Dolostones in Middle Permian Maokou Formation in the Sichuan Basin contain abundant natural gas resources but their origins are controversial for years. The dolostones are present in the lower- to middle-parts of the Member 3 of Maokou Formation, with thickness ranging from 3.0 to 46.8 m. The lateral distribution of Maokou dolostone extends as a NW-SE belt along a basement fault system. Field and drilling core observations show that the dolostones contain numerous tension fractures,fully-or partly-filled by coarse dolomite and calcite as veins,sometimes presenting a breccia structure. Four textural constituents are identified under the microscope: (Ⅰ)Silt-to-fine crystalline scattered euhedral dolomite floating in lime mud matrix; (Ⅱ)Fine crystalline planar-s dolomite(mostly subhedral crystals packed as a mosaic texture); (Ⅲ)Fine-to-coarse crystalline planar-e dolomite with “cloudy center”and “clear rim”(sucrose texture with high intergranular porosity); (Ⅳ)Coarse saddle dolomite in veins. BSE(Backscattered Electron)images confirmed that there are lots of micropores and tiny calcite residuals on crystal surface of “cloudy center”dolomite,while type Ⅲ “clear rim”dolomite and type IV dolomite in veins reveal clean and smooth crystal face without micropores,indicating an origin of authigene during dolomitization. All the four types of dolomite show similar features of rich in calcium,poor in iron,manganese and strontium,but their content of MgO varies differently. MgO(%)of type I and type II dolomite is always lower than that of stoichiometry dolomite,however,MgO(%)in type Ⅲ and type IV dolomite is close to or even higher than the stoichiometry. Lithological,mineralogical and geochemical features indicate two episodes of dolomitization for the formation of Maokou dolostones. Type Ⅰ, type Ⅱ and type Ⅲ “cloudy center”dolomite were formed during the first episode of dolomitization;and type Ⅲ “clear rim”and type Ⅳ vein dolomite were authigenic during the second episode of dolomitization. Both episodes of dolomitizations started very early,possibly from synsedimentary to shallow burial period. In situ analyses on strontium isotopes and rare-earth elements suggest that both dolomitizations were related to the activities of Emeishan mantle plum eruptions. The dolomitic fluids were featured by mixing sources from both magmatic related hydrothermal fluid and Ca-bearing underground water. The tension fractures were proved to be effective channels for fluids mixing and migrating to the surrounding sediments.