鄂尔多斯盆地天环坳陷北段桌子山组白云岩碳氧同位素特征及意义

Characteristics and implications of carbon and oxygen isotopes in dolomites of the Ordovician Zhuozishan Formation in northern Tianhuan Depression,Ordos Basin

通过空间展布和不同类型白云岩的碳氧同位素特征,诠释了鄂尔多斯盆地西部天环坳陷北段奥陶系桌子山组白云岩的成因和储层地质学意义。研究区白云岩主要有微—粉晶白云岩、残余砂屑细晶白云岩和细—中晶白云岩3种,其中微—粉晶白云岩δ^(13)C、δ^(18)O平均值为0.78‰和-5.0‰;残余砂屑细晶白云岩δ^(13)C、δ^(18)O平均值为0.53‰和-6.5‰;细—中晶白云岩δ^(13)C、δ^(18)O平均值为0.47‰和-6.3‰。桌子山组白云岩碳氧同位素的组成位于正常海相碳酸盐岩范围之内,说明其白云石化流体来源于囚禁的古海水。白云岩碳氧同位素演化特征及成岩作用特征,指示微—粉晶白云岩为准同生阶段白云石化的产物,而残余砂屑细晶白云岩及细—中晶白云岩则为浅埋藏白云石化的产物。

The origin and geological significance of dolomites in the Ordovician Zhuozishan Formation in the northern Tianhuan Depression of the western Ordos Basin were interpreted based on spatial distribution and the characteristics of carbon and oxygen isotopes of different types of dolomites.There are three main types of dolomites in the study area,including powder-microcrystalline,residual sand-fine crystalline and fine-medium crystalline ones.The δ^(13)C values of powder-microcrystalline,residual sand-fine crystalline and fine-medium crystalline dolomites are 0.78‰,0.53‰,and 0.47‰,while the δ^(18)O values are-5.0‰,-6.5‰ and-6.3‰,respectively.These values belong to normal marine carbonates,indicating that the dolomite fluid might come from ancient seawater.Carbon and oxygen isotope evolution and diagenetic effects showed that the powder-microcrystalline dolomites were formed during the quasi-synchronic stage,and the residual sand-fine crystalline and finemedium crystalline dolomites were formed during the shallow burial stage.