塔里木盆地西北缘寒武系中热水白云石团块特征及成因研究  

Character and Origin of Cambrian Hydrothermal Dolomite Conglomeration in the Northwestern Margin of Tarim Basin

作　　者：张军涛 胡文瑄[2] 王小林[2] 钱一雄[1] 吴世祥[1] 

机构地区：[1]中国石油化工股份有限公司石油勘探开发研究院,北京100083 [2]南京大学地球科学系,南京210093

出　　处：《地质学报》2011年第2期234-245,共12页Acta Geologica Sinica

基　　金：国家基础研究发展规划"973"项目(编号2005CB422103;2005CB422107);国家科技重大专项(编号2008ZX05005002)资助成果

摘　　要：在塔里木盆地西北缘寒武系中,发育有团块状的热水白云石,其具有明显的层控性:产出在中寒武统顶到上寒武统底的蒸发岩向白云岩过渡段,且仅赋存于粉晶白云岩中。热水白云石具有典型的鞍状白云石特征:弯曲的晶面,波状消光,边缘多发生去白云岩化。地球化学分析显示,其Ca/Mg值稍高于基质白云石,Sr、Fe、Mn和Ba含量也略高于基质白云石。δ18O值(-10.1‰～-12.2‰)较基质白云石(-6.1‰～-7.8‰)明显偏负。不同层位热水白云石的δ13C值之间存在较大的差异,但与同一层位的热水白云石和寄主岩石的δ13C值非常相近。包裹体均一温度多分布在90～120℃之间。综合分析表明,热水白云石形成时的较高温度主要是由较深的埋藏深度和较高的地温梯度所造成的。形成热水白云石的流体并不是岩浆热液,而主要为寒武系白云岩内地层水,并可能有深循环的大气降水掺入,这些流体经历了长期的水岩反应,具有相应的地球化学特征。在断裂活动等作用的驱使下,热流体层内运移。运移过程中,溶解了岩层中硬石膏结核,形成溶蚀孔隙;而当压力降低,CO2大量析出时,开始沉淀形成热水白云石。分析这类热水白云石的形成机制,对于研究塔里木盆地碳酸盐岩储层发育具有一定启示意义。Hydrothermal conglomerated dolomite developed mainly in the Cambrian in the northwestern margin of the Tarim Basin and show distinct strata-bound feature.Hydrothermal conglomerated dolomite is hosted only in crystalline dolomite,the transitional zone from the top of the Mid-Cambrian to evaporate at the bottom of the Upper Cambrian to dolomite.Hydrothermal dolomite is characterized by saddle crystal morphology,curved crystal and wavy extinction in thin section under crossed-polarized light.Geochemical analysis shows that hydrothermal dolomite has higher Ca/Mg values and Sr contents,slightly higher Fe,Mn and Ba contents,and more depleted δ^18O values（-10.1‰--12.2‰）than those of matrix dolomite（-6.1‰--7.8‰）.The δ^13C values of hydrothermal dolomite in different locations vary greatly,but are similar to those of related host dolomite.The homogeneous temperatures of primary inclusions in the dolomites range from 90℃ to 120℃.The field geometry,petrography and geochemistry of hydrothermal dolomites indicate that higher temperature at which hydrothermal dolomite deposited resulted from the depth of deep burial and temperature gradient of the earth.The fluids were not magmatic but formation water from the Cambrian dolomite,possibly with involvement of deep circular meteoric water.After long-term fluid-rock reaction,these fluids were armed with the same geochemical features.Driven by faulting,hydrothermal fluids migrate along the interlayers.During this process,anhydrite nodule in the dolomite is dissolved to form voluminous of dissolved porosity.With the temperature and pressure decreasing and a large amount of CO2 released,hydrothermal dolomite starts to precipitate.Study of the forming mechanism of hydrothermal dolomite of this kind has some significance for understanding of evolution of the carbonatic rock reservoir in the Tarim Basin.

关 键 词：热水白云石 岩石学特征 地球化学特征 流体来源 寒武系 塔里木盆地西北缘 

分 类 号：P588.245[天文地球—岩石学]