摘要
鄂尔多斯盆地及周缘中元古界蓟县系白云岩中广泛发育硅质,硅质被认为是这套白云岩致密化的主要因素。但对硅质赋存特征及其矿物相态关系缺乏详细研究,进而影响了对这套硅质白云岩的硅化和致密化机理的合理解释。野外剖面观测和室内岩石学及地球化学研究表明,白云岩中赋存的硅质以层纹状/条带状、结核状/团块状和弥散状产出。层纹状/条带状硅质以大规模硅质和白云岩相互叠置发育为特征,微观光学显微镜下表现为隐/微晶石英,具有低稀土元素含量、弱Ce负异常、微弱Eu正异常、高Al_(2)O_(3)和TiO_(2)含量等特征,是由海水中SiO_(2)在大陆边缘浅水区域直接沉淀和海水中微生物诱导SiO_(2)在含氧光合生物蓝细菌周围进行沉淀形成。结核状/团块状硅质常以不规则的形式分布在基岩中,主要硅质相态为玉髓、微晶石英,其LaN/CeN值接近1,伴随着弱Ce负异常、微弱Eu负异常,是由海水中的SiO_(2)和陆源碎屑输入的SiO_(2)在晚成岩阶段充填于铸模孔、格架孔形成。弥散状硅质在宏观上不具明显组构特征,微观光学显微镜下以隐/微晶石英为主,具有较高Eu正异常的特征,是由SiO_(2)对白云石进行交代形成。蓟县系白云岩中硅化具有明显的选择性和旋回性,从而导致该套硅质白云岩致密化,致使岩石孔隙度降低,对优质储层的发育不具有建设性作用。
Siliceous dolostone from the Middle Proterozoic Jixian System is broadly distributed in the Ordos basin and adjacent areas.Although silica is well-known as the principal factor tightening the dolostone,less attention has been paid to its various characteristics and the relationships between mineral phases.This lack of understanding has hindered the rational interpretation of the silicification and densification mechanisms of these rocks.Field observations,along with petrological and geochemical studies,reveal three distinct forms of silica within the dolostone:layered/banded,nodular/agglomerated,and dispersed.Layered and banded silica formations involve large-scale amalgamation of silica and dolostone.Microscopically,these formations consist of cryptocrystalline/microcrystalline quartz,with low rare earth element content,faint negative Ce anomalies,subtle positive Eu anomalies,and elevated Al_(2)O_(3)and TiO_(2)content.They originate from direct silica precipitation in shallow continental margin waters,facilitated by oxygen-rich,photosynthetic cyanobacteria induced by marine microorganisms.Nodular/agglomerated silica has an irregular spatial distribution within the bedrock matrix.It is predominantly composed of chalcedony and microcrystalline quartz.The LaN/CeN ratio approximates unity,accompanied by subtle negative anomalies in Ce and Eu.These entities form during late diagenesis through silica infusion from seawater and terrestrial detritus,filling voids such as mold and framework cavities.Dispersed silica lacks distinctive structural attributes.Under microscopic observation,it is mainly composed of cryptocrystalline/microcrystalline quartz.Importantly,it presents a conspicuous and elevated positive Eu anomaly,resulting from the metasomatic replacement of dolomite with silica.The silicification in the Jixian System dolostone displays selective,cyclical patterns,resulting in compaction and reduced rock porosity.However,these processes do not contribute positively to the formation of high-grade reservoirs.
作者
赵青
刘波
魏柳斌
白海峰
芦飞凡
吴淳
石开波
ZHAO Qing;LIU Bo;WEI Liubin;BAI Haifeng;LU Feifan;WU Chun;SHI Kaibo(School of Earth and Space Sciences,Peking University,Beijing 100871,China;Exploration and Development Institute,Changqing Oil Field Branch Petrochina Company Limited,Xi'an,Shaanxi 710018,China)
出处
《地质学报》
EI
CAS
CSCD
北大核心
2024年第10期3117-3133,共17页
Acta Geologica Sinica
基金
中国石油长庆油田公司科技攻关项目“鄂尔多斯盆地中元古界-奥陶系构造沉积演化、成源机制与勘探新领域”(编号ZDZX2021-04)资助的成果。
关键词
鄂尔多斯盆地
中元古界
蓟县系
硅质条带
硅化
Ordos basin
Mesoproterozoic
Jixian System
siliceous rock
silicification
