鄂尔多斯盆地奥陶系白云岩成岩相地球化学特征*

白云岩是鄂尔多斯盆地奥陶系海相碳酸盐岩层系最主要的岩石类型和天然气藏的主要储集层。文中探讨了鄂尔多斯盆地奥陶系马家沟组白云岩成岩相的岩相与地球化学特征及其识别方法。根据岩心及露头观察、薄片鉴定、全岩X衍射、元素分析和碳、氧、锶同位素测试数据,识别出8种白云岩成岩相类型(准同生云化相、膏盐溶角砾相、活跃回流云化相、隐伏回流云化相、海水云化相、表生岩溶角砾相、埋藏溶蚀相和埋藏重结晶相),其分别形成于蒸发海水、中等盐度海水、正常海水、早期淡水、表生淡水及地层水成岩环境,隶属于近地表、风化壳及中—深埋藏3个成岩系统。建立了Fe-Mn、Cs-Sc、(K×Tl)/Na-(Sc×Ni)/Sr、δ¹³C-⁸⁷Sr/⁸⁶Sr 成岩相判别模版。弱活动性元素Sc、Ni和稳定重同位素¹³C、¹⁸O富集于蒸发海水类成岩相,较活动性元素Cs富集于中等盐度及正常海水类成岩相;K、Tl富集于还原环境成岩相,Fe、Mn富集于氧化环境成岩相;表生类成岩相以富含大量吸附态分散元素和 ⁸⁷Sr 为特征。研究结果表明,近地表及风化壳成岩系统决定了研究区马家沟组白云岩的成岩相及其地球化学特征,且岩相学研究及地球化学实验是开展白云岩成岩机理研究的有效手段。

Geochemical characteristics of dolostone diagenetic facies of the Ordovician in Ordos Basin

The dolostone, as the dominant rock type in carbonate rocks of the Ordovician Majiagou Formation in Ordos Basin, is the main reservoir for natural gas. This paper discussed the lithofacise, geochemical characteristics and detecting methods of dolostone diagenetic facies of the Ordovician Majiagou Formation in Ordos Basin. Based on core and outcrop observation,thin section identification,X-ray diffraction test,elemental analysis,the stable carbon and oxygen isotope test and strontium isotope test,eight kinds of dolostone diagenetic facies were recognized,which formed in the evaporation,poikilohaline,and normal seawater,as well as early and epigenic freshwater,formation water diagenesis environment respectively, and belong to near-surface,weathering crust and deep-buried diagenetic system. The new templates of Fe-Mn,Cs-Sc,(K×Tl)/Na-(Sc×Ni)/Sr,δ¹³C-⁸⁷Sr/⁸⁶Sr are created to recognize dolostone diagenetic facies effectively. The evaporated seawater diagenetic facies is rich in immobile elements like Sc,Ni,as well as¹³C and¹⁸O. The poikilohaline and normal seawater diagenetic facies is rich in mobile elements like Cs. The reductive environment diagenetic facies is rich in K and Tl,and the oxidation environment diagenetic facies is rich in Fe and Mn. The epigenic diagenetic facies is characterized by a great deal of adsorbed disperse elements and ⁸⁷Sr. The above results show that the near-surface and weathering crust diagenetic system are key factors to decide lithofacies and geochemical characteristics of dolostone diagenetic facies of the Majiagou Formation in study area,and the microscopic petrography and geochemistry experiment are effective means of research on diagenetic mechanism of dolostone.