鄂尔多斯盆地南部奥陶系马家沟组中下组合白云岩稀土元素特征及其成因

The REE characteristics and genesis of the middle and lower combinations dolomite of Ordovician Majiagou in the southern Ordos Basin

基于岩石学及稀土元素等特征,开展马家沟组中下组合白云岩储层成因机制研究。结果表明,研究区主要发育泥微晶、粉细晶和细中晶3种类型的白云岩;不同类型白云岩的稀土元素总含量具有一定的差异性,整体含量较低,平均值为13.75×10^-6,Y/Ho平均值为38.4,主要与沉积与埋藏作用有关;白云岩稀土元素的配分模式均与泥晶灰岩相似,轻稀土元素相对富集,重稀土元素相对亏损,NdN/YbN的平均值为6.87,δCe值均表现为显著负异常;不同类型白云岩稀土元素Eu异常值具有一定的差异性,泥微晶白云岩δEu值表现为微弱正异常,平均值为1.02,粉细晶白云岩和细中晶白云岩表现为微弱负异常,平均值均为0.96,且多数样品Y和La呈显著正异常,表明泥微晶白云岩形成于相对开放、低氧化性、低温度的局限台地环境,粉细晶白云岩形成于相对封闭、低还原性、弱—中等水动力条件的潮缘浅滩环境,细中晶白云岩形成于相对封闭、低还原性、温度相对较高的潮坪浅滩环境,且主要受回流渗透白云石化作用机制控制。

Based on the characteristics of petrology and rare earth elements,the genetic mechanism of the middle and lower combined dolomite reservoirs in the Majiagou Formation was studied.The results show that there are mainly three types of dolomite,namely micritic dolomite,silty-fine dolomite,fine mesocrystalline dolomite in the study area.The total content of rare earth elements in different types of dolomites is somewhat different,and the overall content is relatively low.The average value is 13.75×10^-6 and the average value of Y/Ho is 38.4,which is mainly related to sedimentation and burial,the REE pattern of dolomite is similar to that of limestone,the light rare earth elements are relatively enriched,the heavy rare earth elements are relatively depleted,the average of NdN/YbN is 6.87,and theδCe values are all significant negative anomalies,theδEu values of different types of dolomite have some differences.TheδEu values of micritic dolomite is negative anomalies,with average values of 1.02,respectively.The silty-fine dolomite and fine mesocrystalline dolomite exhibits a weak negative anomaly with an average of 0.96,and most of the samples Y and La presented significant positive anomalies.It indicates that the micritic dolomite is formed in a relatively open,low-oxidation,low-temperature limited mesa environment,and the silty-fine dolomite is formed in a relatively closed,low-reduction,weak-medium hydrodynamic condition in the tidal shoal environment,fine medium crystal Dolomite is formed in the relatively closed,low-reducing,relatively high temperature tidal flat shoal environment,which is mainly controlled by the mechanism of reflux infiltration and dolomitization.