白云岩化作用类型及机理——以歧口凹陷古近系沙河街组一段为例

通过镜下鉴定和地球化学分析对歧口凹陷古近系沙一段白云岩进行了研究,综合分析了研究区发育的岩石类型及其成因。结果表明:研究区发育泥-微晶白云岩、灰质白云岩、白云质灰岩及少量颗粒白云岩;Sr/Ba值为0.44.9,V/Ni值为1.37.1,Z值(盐度指数)分布在122.6162.5;87Sr/86Sr值和同期海水极为接近,因此推断湖盆遭受了海侵作用,海侵为白云岩化作用提供了Mg~(2+);有序度分布在0.22 0.53,整体较低;Mg/Ca值为0.730.91,反映了富钙低镁的特征,表明白云岩形成于准同生阶段。碳氧同位素分析表明,研究区白云岩形成于浅埋藏环境,并且受到了发酵作用的影响,异常高的Sr值也反映了微生物活动的存在。通过以上证据推断本区白云岩是在准同生期、浅埋藏环境、经渗透回流作用形成的,并且受到了发酵作用的影响。

Type of Dolomization and Formation Mechanism:A Case Study of Palaeogene Es1 in Qikou Sag

In terms of the microscopic identification and geochemical analysis,study has been made on the dolostones of Palaeogene Es1 in Qikou sag.The result shows that there are microcrystallinemicritic dolomite,calcite dolomite,dolomite limestone,and a little grain dolomite.Sr/Ba ratio is from 0.4 to 4.9,V/Ni ratio is from 1.3 to 7.1,Z value distributed between 122.6～162.5,87Sr/86Sr ratio of dolostone is extremely close to the seawater.According to the mentioned values,the lake basin was influenced by transgression activity,which brought a large number of Mg2+ for dolomization.The order degree of dolomite is between 0.22-0.53,Mg/Ca ratio is between 0.73-0.91,it has characteristics of rich in Ca2+ and poor in Mg2+,which indicates that the dolostone was formed during the penecontemporaneous period.Carbon and oxygen isotope analysis implies that the dolostone was formed in shallow burial environment,the extremely positive δ13C anomaly indicates that the process of dolomization was affected by fermentation,the abnormal high Sr value suggests an activity of bacteria.Based on the analysis above,the dolostone was formed in shallow burial environment during the penecontemporaneous period by the process of seepage-reflux and the process was affected by methanation.