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鄂尔多斯盆地西缘奥陶系白云岩 地球化学特征及成因分析

作    者:吴兴宁1,2, 吴东旭1,2, 丁振纯1,2, 于 洲1,2, 王少依1,2, 李维岭1,2
单    位:1中国石油杭州地质研究院; 2中国石油集团碳酸盐岩储层重点实验室
基金项目:本文受国家科技重大专项课题“寒武系—中新元古界碳酸盐岩规模储层形成与分布研究”等资助
摘    要:
白云岩成因及白云岩储层发育的控制作用尚不明确,这造成了优质白云岩储层预测的困难,严重制约了鄂尔多斯盆地西缘奥陶系的勘探部署和天然气发现。早奥陶世,盆地西缘总体处于开阔海缓坡或弱镶边台地沉积环境,内缓坡及台地边缘带起到一定的障壁作用。岩石薄片观察表明,盆地西缘奥陶系白云岩主要为颗粒白云岩和晶粒白云岩,颗粒白云岩残余颗粒结构清晰,晶粒白云岩包括泥粉晶白云岩、粗粉晶白云岩、粉细晶白云岩、细晶白云岩、中细晶白云岩和中粗晶白云岩等。白云岩地球化学分析显示:泥粉晶白云岩碳同位素δ13C值多偏正,微量元素具有低Na、K及较高Fe的特征,锶同位素87Sr/86Sr值与同时期海水相似,稀土元素Eu亏损,阴极发光显示为暗棕色或暗褐色;细晶-颗粒白云岩δ13C值多偏负,微量元素具有低Na、K、Fe的特征,87Sr/86Sr值明显大于同时期海水,阴极发光显示为桔黄色、暗红色及红色发光,包裹体均一温度普遍偏高。根据区域构造和沉积演化、岩石学特征、气候环境以及地球化学特征综合分析,盆地西缘奥陶系泥粉晶白云岩由准同生期蒸发泵白云石化作用形成,细晶-颗粒白云岩由浅埋藏期回流-渗透白云石化作用形成。同生期颗粒滩原生孔隙在白云石化之后由于白云岩的抗压实能力强而得以保存下来,后期进一步溶蚀扩大形成现今盆地西缘奥陶系白云岩储层中最主要的储集空间。
关键词:白云岩; 地球化学特征; 成因; 桌子山组; 克里摩里组; 奥陶系; 鄂尔多斯盆地

Geochemical characteristics and genetic analysis of Ordovician dolomites in the western margin of Ordos Basin

Author's Name: WU Xingning, Wu Dongxu, DING Zhenchun, YU Zhou, WANG Shaoyi, LI Weiling
Institution: 
Abstract:
The genetic model and the controlling factor on the development of Ordovician dolomite reservoir are not clear, which seriously restricted the effective prediction of high quality dolomite reservoir, and the further exploration and discovery of the natural gas in the western margin of Ordos Basin. Controlled by the evolution of Helan depression in Early Ordovician, the western margin of Ordos Basin is located in a gentle slope or the weakly-rimmed platform of open sea with a certain barring of the inner gentle slope, middle gentle slope and the platform edge belt, where carbonate rocks extensively developed. The observation of rock and thin section shows that the Ordovician dolomites in the western margin of the basin are mainly composed of granular dolomite with clear residual particle structure, and crystalline dolomite including micritic-powder crystalline dolomite, coarse powder crystalline dolomite, powder-fine crystalline dolomite, fine crystalline dolomite, fine-medium crystalline dolomite, medium-coarse crystalline dolomite. The geochemical analysis of dolomite shows that the oxygen isotope δ18O values of the Ordovician dolomite in the western margin of the basin are generally negative, the carbon isotopes δ13C values are more positive, the trace elements are characterized with low Na, K and higher Fe content, and the strontium isotope 87Sr/86Sr values are obviously larger than those of the seawater in the same period, and the REE distribution curve is right deviation model of negative Eu anomaly. The cathodoluminescence shows orange, dark red and red, and the homogenization temperature of inclusions is generally high. According to comprehensive analysis of petrological characteristics, regional tectonic and sedimentary evolution, climatic environment and geochemical characteristics, the Ordovician micritic-powder crystalline dolomite in the western margin of the basin was formed by evaporation pump dolomitization in the paracontemporaneous period, and the fine crystalline-granular dolomite was formed by reflux infiltration dolomitization in shallow burial period. The inner gentle slope, middle gentle slope and the granular shoals on the edge of the platform have high primary porosity, which are more easily dolomitization, and the distribution of the Ordovician dolomite is discontinuous and uneven. After dolomitization, the primary pore of graualr shoal in contemporaneous period was preserved because of the strong anti-compacting ability of dolomite. Later, further dissolution expanded to form the most important reservoir space in the Ordovician dolomite reservoir.
Keywords: dolomite; geochemical characteristics; origin; Zhuozishan Formation; Kelimoli Formation; Ordovician; Ordos Basin
投稿时间:2020-12-22  
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