塔里木盆地寒武系—奥陶系白云岩稀土元素特征及其成因意义

Rare Earth Element Geochemistry Characteristics of Cambrian-Ordovician Dolostones in the Tarim Basin and Their Implications for the Origin

塔里木盆地寒武系—奥陶系白云岩是重要的油气储集层,按能反映成因的结构构造特征将其分为4类,即泥晶—粉晶白云岩、藻纹层白云岩、斑状白云岩及重结晶白云岩。通过分析各类白云岩的稀土元素组成特征,并结合岩石学研究,探讨了各类白云岩成岩流体性质及其水岩比的相对大小。4类白云岩的ΣREE差别较大,但稀土元素配分模式均具LREE轻微富集、HREE亏损、Eu负异常且与海相泥晶灰岩相似的特征,其稀土元素配分模式主要受海水的影响。4类白云岩经受的成岩作用差别明显,但REE迁移行为类似,无明显分馏。泥晶—粉晶白云岩中ΣREE平均值为51.55×10-6,与泥晶灰岩相近,其ΣREE主要受控于原岩,受后期成岩作用的影响相对较小,其成岩过程的水岩比较小。藻纹层白云岩的ΣREE平均值为21.37×10-6,不及海相泥晶灰岩的1/2;岩石学及地球化学资料反映其成因与藻类密切相关,其低ΣREE可能是由ΣREE极低的藻类参与白云岩化所致,后期成岩改造作用对其影响并不明显。斑状白云岩的ΣREE平均值为28.89×10-6,远低于泥晶灰岩及泥晶—粉晶白云岩,反映其白云岩化过程的水岩比远高于泥晶—粉晶白云岩。重结晶白云岩的ΣREE平均值为13.64×10-6,不及泥晶灰岩的1/4,在4类白云岩中最低,岩石学特征表明此类白云岩经受了白云岩化及后期强烈重结晶2期主要成岩作用,而这2期成岩作用均需极高的水岩比,此类白云岩的稀土元素组成特征主要是这2期成岩作用叠加的结果,其低ΣREE值可能是大量ΣREE含量极低的成岩流体改造的结果。

Cambrian-Ordovician dolostones in the Tarim Basin are hydrocarbon reservoir rocks of vital importance.Based on detailed petrographic investigations,four types of dolostones have been recognized,which are,respectively,mud-silt-sized dolostone,algal laminated dolostone（ALD）,prophyritic dolostone,and recrystalized dolostone.Determination of rare earth element（REE） composition,combined with petrographic observation,has been employed aiming to gain insights into the property of diagenetic fluids and fluid-rock ratios.The ΣREE of the four types of dolostones distinctly differentiates from each other.However,their REE patterns are all rich in LREE,depleted in HREE and have Eu negative anomalies.These characteristics are comparable to those of finely crystalline limestone,and are mainly influenced by the sea water.These four types of dolostones show similar REE mobility behaviour and no significant fractionation,although they have been subjected to evidently different diageneses.Mud-silt-sized dolostone has an average ΣREE of 51.55×10^-6,close to that of the finely crystalline limestone,suggesting the REE primarily inherited from its precursor limestone,and the dolostone probably experienced relatively low fluid-rock ratio during diagenesis.The ΣREE of ALD is 21.37×10^-6 which less than 1/2 of that of the mud-silt-sized dolostones.The petrographic and geochemical characteristics reveal that algae,which has very low ΣREE,possibly played an important role during its diagenesis.So the low ΣREE of ALD was likely resulted from the involvement of algae during the dolomitization,further more,it was not modified significantly during epigenetic diagenesis.The ΣREE of prophyritic dolostone is 28.89×10^-6,much less than that of the finely crystalline limestone and mud-silt-sized dolostone,indicating that it experienced diagenesis of higher fluid-rock ratio than the mud-silt-sized dolostone.The ΣREE of recrystalized dolostone is 13.64×10^-6,less than 1/4 of that of finely crystalline limestone,and ranks the lowest in the 4 types.Petrographic and geochemical characteristics show that this kind of dolostone formed as results of dolomitization and recrystallization of pre-existing dolomite during deep burial diagenesis,both of which require high fluid-rock ratios.The REE composition characteristics were probably caused by dolomitization and superimposed by recrystallization of pre-existing dolomite.