松辽盆地营城组中基性火山岩成岩作用:矿物晶出序列、杏仁体充填和储层效应

Diagenesis of intermediate and mafic volcanic rocks of Yingcheng Formation(K_1y) in the Songliao basin;Sequential crystallization,amygdule filling and reservoir effect

利用露头和钻井资料,通过显微镜观测、化学分析和电子探针分析,研究营城组中基性火山岩的矿物晶出系列和杏仁体充填及其储层效应。本区中基性火山岩包括玄武岩、安山岩、玄武安山岩、粗面玄武岩、玄武粗安岩和粗安岩。斑晶矿物的晶出顺序为:橄榄石最早并几乎全部蚀变为蛇纹石、伊丁石和磁铁矿,仅保留橄榄石假象;辉石被斜长石包含或与之交生,说明辉石晶出有的略早于斜长石、有的与之同时。基质呈间隐间粒结构,为比斑晶偏酸性/碱性的板条状微晶长石堆积搭成格架、内充填玻璃质及微晶粒状矿物(橄榄石、辉石和磁铁矿)。基质结晶晚于斑晶,晶出序列为微晶粒状矿物→微晶长石→玻璃质。有单成分和复成分两种杏仁体。单成分者主要见硅质和钙质,具结晶世代性,为后生流体沉淀充填形成,可作为储层变差或非储层的标志。复成分杏仁体是原生火山玻璃固态下水合与蚀变作用的结果:包括蛇纹石/绿泥石-火山玻璃、石英-方解石-皂石/方解石和石英-绿泥石-方解石等组合类型,通常可作为储层改善或有效性增加的标志。中基性火山玻璃蚀变分为四个阶段:新鲜火山玻璃(折光率1.57)→水合火山玻璃(折光率降至1.53)→橙玄玻璃(铁镁质微晶矿物集合体)→新生矿物(石英、方解石、蛇纹石、绿泥石、皂石);由火山玻璃变为新生矿物的体积减小或孔隙增加效应为7%～10%。

Crystallizing sequence,amygdule filling features and reservoir effect were studied on the intermediate and mafic volcanic rocks of K_1y in the Songliao basin by means of microscope,chemical determination and electron probe.The volcanics include basalt, andesite,basaltic andesite,trachybasalt,basaltic trachyandesite and trachyte.Crystallizing sequence of the porphyritic crystals is as follows.The earliest is olivine(Ol) which is totally altered into serpentite(Srp),iddingsite(Idn) and magnetite(Mag) left a frame of the olivine.Pigeonite(Pgt) crystallized simultaneously with or a little bit earlier than plagioclase(Pl) because Pgt inclosed/ intergrowed with Pl.Following the porphyritic crystals,matrix is composed of fine-grained piled-up Pl needles infilled with micro-grains of Ol,Pgt and Mag as well as glass,called tholeiitic texture.Crystallizing sequence of the matrix is Ol/Pgt/Mag→Pl→glass.There are two kinds of amygdule,mono-and multiple component ones.The mono is silica or calcite with growing rings showing increasingly sized particles inwards,which is preferred to be interpreted as sequential precipitation of a post fluid.This can deteriorate the reservoir quality.The multiple ones involve in the associations of Srp/chlorite(Chl)-glass,quartz(Qtz)-calcite(Cal)-saponite(Snt),and Qtz-Chl-Cal.They are made up of hydration of glass in solid state through four steps:fresh glass(refractive index 1.57),hydrate glass (refractive index 1.53),palagonite and new formed minerals mainly of Qtz,Cal,Chl and Snt.As a result of the devitrification and recrystallization,secondary pore-spaces can be increased by 7%to 10%,improving the reservoir features of the volcanics.