鄂尔多斯地区奥陶系马家沟群中部块状白云岩的深埋藏白云石化机制

Mechanism of Deep Burial Dolomitization of Massive Dolostones in the Middle Majiagou Group of the Ordovician,Ordos Basin

鄂尔多斯地区奥陶系马家沟群中部发育一套厚层块状白云岩。在西部地区定边至鄂托克旗一带 ,这套块状白云岩连续厚度达数百米。定探 1井揭示的最大厚度为 431m。自西向东 ,白云岩体由厚变薄 ,并与石灰岩呈指状交织。这套块状白云岩由粗粉晶—细晶白云石组成 ,斑状结构发育。常见云斑的边界为缝合线 ,这表明斑状白云石化明显受缝合线控制。白云岩有序度平均值为 0 .85 ,CaCO3 摩尔含量平均为 5 0 .6 5 g/mol。δ13 C的平均值为 0 .6‰ (PDB) ,δ18O的平均值为 - 7.4‰ (PDB)。主体细晶白云石中液态包体常见 ,最低均一温度为 10 4℃ ,包体成分中含有大量的有机成分CH4 和无机成分H2 S。上述特征表明 ,这套白云岩是由深埋藏白云岩化作用形成的 ,它显示出良好的储集性能 ,是重要的后备储集岩。

There is a suit of massive dolostones(as MD hereafter)which develop in the middle Majiagou Group of the Ordovician in the Ordos basin.It is mainly distributed in Majiagou Fm.Ⅳ,and it has several hundred metres at the zone of Dingbian County to Eteke Qi in the west part of Ordos basin, with maximum thickness 431 m in Dingtan Well 1.It becomes thinner and thinner from west to east,and is intercalated with limestones from the middle to eastern part of Ordos. The MD consist of coarse silt-sized to fine sand-sized dolomite crystals.The mottled texture is well developed in such dolostones,and the boundary of dolomitic spots is commonly stylolites.It shows that mottled dolomitization is obviously controlled by stylolites. The dolomites have uniform dar red colour under cathodoluminecence. The analytical data of 67 samples shows the ordering of dolomites averages 0.85,and the molar concentration of CaCO 3 averages 50.65%;δ 13 C averages 0.6‰(PDB),δ 18 O averages -7.4‰(PDB).The characteristics of cathodoluminecence and X-ray indicate approximate ideal dolomites are formed in stable diagenetic environment of slow conclear and crystalization .The homogeneous temperature of 21 primary inclusions in the host crystals is from 104℃ to 355℃(uncorrected by pressure),and the temperature of 104℃ is the lowest one.According to the earth temperature gradient of 3.98℃/100m in this area ,the maximum burial depth of dolomitization is at least 2 600 m. Besides,the composition of inclusions has two parts-organic and inorganic and there is a certain of CH 4 both in gas phase and in liquid phase. All characteristics above prove that the huge thick dolostone body is formed in deep burial dolomitic environment.