塔里木盆地中央隆起带寒武系-奥陶系白云岩成岩相及其地球化学特征

Diagenetic facies of dolomite and geochemical characteristics across the Cambrian-Ordovician transitions in the Central Uplift Zone,Tarim Basin

成岩相识别与划分有助于碳酸盐岩储层分类评价及预测。为了明确塔里木盆地中央隆起带寒武系-奥陶系的白云岩成岩相类型及特征,通过岩心描述、常规及铸体薄片观察、X衍射分析以及粒度分析等方法,首先对其岩相学特征进行了详细研究并对成岩相类型进行了划分,在此基础上,结合O、C稳定同位素测试方法以及Fe、Mn、Sr、Li、U等常、微量元素测试技术探讨了各类白云岩成岩相的地球化学特征并建立了各类成岩相的模式。研究结果表明,塔里木盆地中央隆起带寒武系-奥陶系白云岩成岩相可划分为准同生云化亚相(PD-F)、浅埋藏隐伏回流渗透云化亚相(SLRD-F)、浅埋藏活跃回流渗透云化亚相(SARD-F)、表生风化壳岩溶亚相(WK-F)和构造破碎亚相(TF-F)共5类。其中,PD-F模式的白云岩云化流体主要来自咸化海水并受部分淡水影响,表现出O同位素负偏,Fe、Mn、Sr和Li质量分数明显偏高的特征;SARD-F模式的白云岩基本形成于正常海水,个别样品受到埋藏溶蚀或重结晶作用以及淡水影响,整体表现为O同位素值与奥陶纪海水一致,Fe、Mn、Sr质量分数相对较低的特点;SLRD-F模式的白云岩形成环境相对封闭,受淡水影响较小,表现为O同位素值与海水一致,Fe、Mn质量分数相对较低,Sr质量分数相对较高的特征;WK-F模式的白云岩样品由于经历表生淡水溶蚀作用和埋藏期的白云化作用,具有O同位素负偏,Fe、U质量分数较高,Sr质量分数较低的特征;TF-F模式的白云岩样品受到构造热液的影响,具有O同位素负偏,Fe、Mn、U、Li质量分数低的特征。

The identification and division of diagenetic facies is useful for the classification,evaluation and prediction of carbonate reservoirs.To clarify the type and characteristics of the diagenetic facies of dolomite near the Cambrian-Ordovician transitions in the Central Uplift Zone of Tarim Basin,in this work,the lithographic characteristics of dolomite were studied in detail,and the category of diagenetic facies was divided by core description,conventional and casting thin section observation,X diffraction analysis and grain size analysis.Based on this,the geochemical characteristics of different diagenetic facies were discussed,and their formation models were established by testing stable isotopes,which included oxygen and carbon isotope as well as major and trace elements(Fe,Mn,Sr,Li and U).The results show that the diagenetic facies of Cambrian-Ordovician dolomite in the Central Uplift Zone,Tarim Basin,can be divided into five types,including penecontemporaneous dolomatization diagenetic facies(PD-F),shallow latent reflux dolomatization diagenetic facies(SLRD-F),shallow active reflux dolomatization diagenetic facies(SARD-F),weathered crust karstification diagenetic facies(WK-F)and tectonic fracture diagenetic facies(TF-F).Among them,the dolomitic fluid of PD-F mode dolostones mainly come from saltwalized seawater and is affected by freshwater,which shows an obviously negative bias of oxygenisotopes and high Fe,Mn,Sr and Li contents.The dolostone formed in the SARD-F mode shows a consistent value with Ordovician sea water and relatively low Fe,Mn,and Sr contents because it mainly formed in normal sea water environment and individually experienced burial dissolution and recrystallization and was affected by freshwater.Because the diagenetic environment of dolostone formed in SLRD-F mode is relatively closed,this dolostone is characterized by consistent values of oxygenisotopes,relatively low Fe and Mn contents but high Sr contents.Dolostone samples formed in WK-F mode have high Fe and U but low Sr contents because they experienced dissolution of freshwater and burial dolomitization.The dolostone formed in the TF-F model is characterized by low Fe,Mn,U and Li because of the influence of structural hydrothermal fluids.