鄂尔多斯盆地下奥陶统马家沟组马五段白云岩的地球化学特征

Geochemical Characteristics of Dolomites in Lower Ordovician Majiagou Formation,Ordos Basin

鄂尔多斯盆地奥陶系马家沟组马五段属于浅海碳酸盐层系,间夹蒸发岩层。盆地中东部通常含有白云岩,它们是主要油气储集层。一些研究者把它们看成近地表回流白云岩。然而白云岩的岩石学和地球化学表明这些白云岩具有埋藏成因的证据。马家沟组白云岩(马五段)具有暗红色阴极发光,微量元素Fe为5500×10-6～6200×10-6,Mn为170×10-6～210×10-6,稳定同位素氧组分为-7.093‰～-9.932‰PDB,(平均值为-8.671‰PDB),稳定同位素锶(87Sr/86Sr)为0.709766～0.70865(有硅质碎屑物的放射性87Sr的影响)。薄片下可以看到白云石沿着裂缝交代,上述特征表明白云岩可能在埋藏条件下形成。白云岩稀土元素的分布模式与海水的稀土元素分布有明显的差别,缺少Ce和Eu的负异常,表明交代流体不会是海水或蒸发海水。包裹体的均一化温度为160～220℃,盐度为5wt%～25wt%,暗示白云石化作用温度高,压力大,交代流体为卤水。密西西比谷型(MVT)矿化作用(方铅矿和闪锌矿)以及共生的低温热液矿物——钾长石、萤石、石英、黄铁矿等表明白云岩具有区域热液特征。

Lower Ordovician Majiagou Formation in Ordos Basin is a shallow marine carbonate sequence with evaporate stratum. The central-western part in the basin often contains several dolomite units, which are major reservoir rock. Some researchers considered them as reflux dolomite near surface. However, petrography and geochemistry of dolomite suggest that this dolomite have evidence of burial origin. Majiagou dolomite （M5 ） included dull-red cathodoluminescence, trace element of 170 - 210 ppm for Mn and 5500 - 6200 ppm for Fe, stable isotopic oxygen composition of - 7. 093 - - 9. 932 PDB, the average is - 8. 671 PDB, for matrix and rug-filling saddle dolomite and bright dolomite cement, stable isotopic strontium composition of 0. 709 766 -0. 708 65 for ^87 Sr/^86 Sr （influenced by silica-clastics materialities, sea water）. Dolomite replaced host rock along the fractures in the section. These data about Majiagou dolomite （ M5 ） was formed in the burial condition. REE pattern of dolomite are distinctly different from sea water, commonly are short of negative anomaly of Eu and Ce, such distinct REE signatures probably reflected interaction between diagenetic fluid and host rock and siliciclastic material. Homogenization temperature of 160℃ - 220℃, salinity of 5wt% - 25wt% , suggest dolomitization occurred at high temperature and larger pressure, replacing fluids is brines. The pervasive dolomite, trace MVT mineral （sphalerites, galenas and iron sulfides）, potassium （K）-silicate mineral （K-feldspar） , quartz, volcanic tufts, in these rock, which preserve a regional hydrothermal signatures. The petrography and geochemistry of Majiagou dolomite show that this dolomite （ M5 ） has evidence of hydrotherreal dolomite （HTD） and central field （gas） are now identified to be of HTD type, typically with a strong structural control on localization.