塔中地区奥陶系白云岩岩石地球化学特征及成因机理分析

Geochemical Characteristics and Genetic Mechanism of the Ordovician Dolostone in the Tazhong Area,Tarim Basin

塔中地区奥陶系白云岩分布广泛,厚度大,是该区重要的储层之一。根据白云石晶体结构大小、原始沉积特征保存情况,将该区白云岩分为泥晶白云岩、藻纹层泥晶白云岩、颗粒白云岩、粗粉晶—细晶白云岩、中晶—粗晶白云岩、斑状白云岩等6种类型。泥晶白云岩、藻纹层泥晶白云岩保留了其原岩沉积特征,地球化学特征表现为:碳、氧同位素偏高,盐度指数略高于正常海水,微量元素锶、钠含量高和铁、锰含量低,稀土元素特征大体相似,将其解释为准同生白云石化作用产物。结晶白云岩和颗粒白云岩具有晶体相对粗大、明显的交代结构(如雾心亮边结构和环带构造)特点,地球化学特征表现为:碳、氧同位素具有随着成岩作用加强依次向负值偏移,盐度指数逐渐降低的特点,微量元素锶、钠含量低和铁、锰含量高,稀土元素总量变化较大,REE配分模式和Ce、Eu异常相似,将其解释为埋藏阶段白云石化作用的产物,并且在白云石化过程中可能叠加有构造热液作用的影响。在白云岩成因机理分析的基础上,建立了适合研究区发育的各种白云岩类型的白云石化作用模式。

The Ordovician dolomite in the Tazhong area is widely distributed, being one of the most important reservoirs in the Tarim basin due to its huge thickness. Based on the size of the crystal structure of dolostone and the original sedimentary features, six types of dolostone have been identified： micritic dolostone, algal-laminated micritic dolostone, granular dolostone, coarse and fine grained dolostone, medium-coarse crystalline dolostone, and mottled dolostone. The first two dolomites preserve the original sedimentary features. They are geochemcially characterized by high C and O isotopic compositions, slightly higher salinity than common seawater, high Sr and Nr, low Fe, roughly same REE pattern, which can be explained as the result of penecontemporaneous dolomitization. Granular dolostone and crystalline dolostone are of relative large crystals, distinct replacement structure, such as zonation structure. Geochemcial features display that C and O isotopic values shift to negative as diagenesis increasing, salinity index decreases gradually, and low Sr and Na, high Mn, varied total REE amount, the same REE distribution pattern as Ce and Eu anomalies, which can be explained by that they are the product of dolomitization during the burial stage and may be affected by structure hydrothermal superimposition during the process of dolomitization. Based on the study of origin of the dolostone, a dolomitation model applied to any type of dolostone in the study area is established.