准噶尔盆地乌夏地区二叠系风城组云质岩类成因研究

摘要

The dolomitic rocks of the Permian Fengcheng Formation are formed under special geologic conditions, and are important hydrocarbon reservoir rocks in Wu-Xia area in the Junggar Basin. Data of petrologic characteristics, SEM EDS and stable isotope composition indicate that dolomitizing host rocks are volcaniclassic rocks. Dolomitizing fluids probably came from brine from nearshore and shallow-water lake deposits in the Fengcheng Formation, which formed under arid climatic conditions, and from residual Mg-rich seawater from underlying Jiamuhe Formation and Carboniferous strata. Tuff has a significant content of plagioclase, in which anorthite and labradorite hydrolysis by CO2 can be couple with calcite precipitation. Later, Mg-rich brine percolate and replace calcite of the first stage to precipitate dolostones. The dolomitizing fluid has been driven by the thermal convection flow generated by volcanic eruption. At the same time, the overthrust of the Wu-Xia growth fault have speeded up the flow of deep Mg-rich water upwards, and induced the water to quickly penetrate and horizontally migrate in the strata. Fracture and fault play important role in controlling the formation of dolostone and the distribution of favorable reservoir.%　　准噶尔盆地乌夏地区风城组发育的云质岩类是一套特殊成因的、重要的油气储层。岩石学、扫描电镜能谱和稳定同位素分析表明，发生白云岩化作用的母岩是火山凝灰物质，白云岩化的流体来自风城组干旱气候条件下形成的咸化滨浅湖卤水以及下伏佳木河组和石炭系残留的富镁海水。凝灰岩中富含大量的斜长石，其中的富钙长石在二氧化碳参与下水解蚀变，沉淀方解石(即方解石化)。后期富镁卤水渗滤早期形成的方解石，成岩交代形成白云岩。富镁流体运移受火山喷发产生的热流驱动，产生热对流环。同时，乌夏断裂同生逆掩活动加快深层富镁流体上侵，并使流体在层间快速渗透、流动。构造作用产生的裂缝或断裂对白云岩的形成及优质储层分布起到决定性控制作用。