准噶尔盆地乌—夏地区二叠系风城组云质岩特征及成因

云质岩储集层是准噶尔盆地乌—夏地区一套成因特殊的、富含油气的碳酸盐岩储集层。在分析乌—夏地区二叠系构造、沉积及火山活动等地质背景的基础上,根据风城组云质岩岩心宏观产状,将云质岩分为4种类型:波纹层状云质岩、团块状云质岩、丝絮状云质岩和星散状云质岩。镜下波纹层状云质岩晶粒细小,晶形差;团块状云质岩、丝絮状云质岩、星散状云质岩中白云石晶体以粉—细晶为主,普遍发育雾心亮边、环带交代残余结构。C、O同位素及微量元素等地化分析表明:白云岩化的成岩流体来自二叠系风城组半封闭湖湾的咸化沉积地层水以及下伏二叠系佳木河组、石炭系上涌的海相卤水。包裹体测温表明,蚀变方解石的形成温度为60～70℃,白云石的形成温度为85～105℃,白云石形成晚于蚀变方解石。波纹层状云质岩是准同生成因;团块状云质岩、丝絮状云质岩、星散状云质岩是后期成岩交代成因,交代的原岩是火山物质(凝灰岩)。成岩交代云质岩的成因机理是:火山凝灰物质中的火山玻璃容易发生水解蚀变,产物为蒙脱石、长石和石英;蒙脱石转化为伊利石、长石水解蚀变均可析出大量的Ca²⁺,结合CO₂或CO₃^2-形成蚀变产物方解石;方解石在富Mg²⁺流体交代下发生强烈白云岩化作用。

Characteristics and origin of dolomitic tuffin the Permian Fengcheng Formation in Wu-Xia area of Junggar Basin

The dolomitic tuff of Fengcheng Formation in Wu-Xia area of Junggar Basin is considered to be a set of hydrocarbon-abundant carbonate reservoir with special geologic origins. Based on the geological background study including tectonic,sedimentation and volcanic activities in the Permian Fengcheng Formation,and combined with the macroscopic occurrence of the dolomitic tuff cores,the dolomitic rocks could be divided into four types:Rippled,massive,flocculent and diffuse dolomitic tuff. The characteristcs of the four types with the dolomitic tuff examined under microscope are:The ripple one has fine-grained and poor crystal shape,but the other three ones have silt-grained and fine-grained,and there is fogged center and clear margin structure and residual annulus alternation structure widely formed. The geochemical analyses of C-O isotopes and trace elements suggest that the fluid which promotes dolomitization may probably come from the sedimentary formation water in Fengcheng Formation, which is from semi-closed shore-shallow lakes and Mg-rich marine seawater upwelling from the Permian Jiamuhe Formation and Carboniferous. The inclusion homogenization temperature indicates that the uniform temperature for calcite is among 60～70℃ while 85～105℃ for the dolomite. The rippled dolomitic rocks are penecontemporaneous,but the massive,flocculent and diffuse dolomitic rocks are metasomatized lately and the original rocks of the metasomatism are volcanic material(tuff). Volcanic glass in the volcanic tuff material is easily to take hydrolysis alteration,the products of which are montmorillonite,feldspar and quartz. Both of the alteration of montmorillonite to illite and feldspar hydrolysis can separate out plenty of Ca²⁺. Then,the combination of Ca²⁺ and CO₂ or CO₃^2- form the alteration products calcite,taking a strong dolomization reaction under the metasomatism of rich magnesium fluid environment later.