四川盆地西北部地区中二叠统栖霞组肉红色白云岩成因浅析

Genesis of pale red dolomite in Middle Permian Qixia Formation,northwestern Sichuan Basin

中二叠世沉积的栖霞组是四川盆地天然气勘探的重点层系,近年来屡获突破。白云岩是该地区栖霞组最重要的储集岩类型,一般呈灰色或浅灰色。近期在四川盆地西北部地区的葛底坝剖面及双鱼石地区钻井岩心中,发现栖霞组存在一种特殊的肉红色白云岩。目前鲜有公开文献对这套肉红色白云岩的宏观、微观特征及成因进行过系统地阐述,而肉红色白云岩邻层往往伴有较厚的孔洞型白云岩储层,其存在可能对该类优质储层的形成环境存在指示意义,具有研究价值。本次研究从野外露头及岩心样品的宏观、微观岩石学特征以及实验分析数据等方面入手,对其进行了系统地分析并得出以下初步认识:①栖霞组白云岩中的肉红色是一种次生色,其形成与铁元素含量较高有关,因为该类样品中检测出黄铁矿和赤铁矿成分;②白云石仅核心部分变色明显,后期胶结物并未受到影响,说明其形成时间较早,可能在准同生期就已形成铁质侵染;③这种白云岩宏观上常常呈角砾状并伴随有岩溶现象,说明其可能形成于古地貌相对较高的位置;④地球化学信息表明,肉红色白云岩的δ^(18)O平均值为-1.54‰,远高于其他岩石的平均值-6.93‰,反映了肉红色白云岩形成时的水体盐度较高,揭示了川西北地区栖霞组沉积期存在局限蒸发环境,有利于渗透回流型白云岩的发育。

As the key exploration target of natural gas in Sichuan Basin,the Middle Permian Qixia Formation,which has made successive breakthroughs in recent years,is always dominated by grey or light grey dolomite of the most important reservoir rock.However,some pale red dolomite in this formation was found from both Gediba profile in northwestern basin and drilling core in Shuangyushi area.For the pale red dolomite,the macro and micro characteristics and genesis have rarely been reported.Since around the dolomite,there often developed thick porous dolomite reservoirs,its presence may indicate a development environment of quality reservoirs.So,based on the macro and micro physical properties and experimental data of outcrop and core samples from this formation,the pale red dolomite was analyzed.Results show that(1)pale red in the dolomite of Qixia Formation is a secondary color,which is related to higher iron content owing to pyrite and hematite detected in these samples;(2)only the color in core part of dolomite changes apparently,but the later cement has not been affected,indicating that the dolomite formed very early and iron infection might have been existed during penecontemporaneous period;(3)macroscopically,the pale red dolomite usually represents brecciated shape and is associated with karstification,also illustrating it developed at a relatively higher ancient landform;and(4)from the aspect of geochemistry,its averageδ^(18)O(VPDB)is-1.54‰,much higher than that(-6.93‰)in other rocks,reflecting higher water salinity when the dolomite was formed,simultaneous revealing that a restricted evaporation environment existed at the deposition of Qixia Formation in northwestern basin is conducive to the development of permeable reflux dolomite.