苏里格气田东区下奥陶统马五_5亚段白云岩成因

Genesis of Dolomite in the Lower Ordovician Ma-5_5 Submember in Eastern Area of Sulige Gas Field

苏里格气田下奥陶统马家沟组马五5亚段白云岩单层厚度大,横向尖灭快,其特征显著区别于靖边气田单层较薄、横向连续性好的白云岩。研究区马五5亚段白云岩以泥晶白云岩、粉晶白云岩和细—中晶白云岩为主,其有序度在总体偏低的背景下呈现与晶体尺寸呈正比的特征。泥晶(藻)灰岩锶含量远小于同时代全球泥晶灰岩的锶含量,锶残留程度受到淡水和白云岩化等多种因素的影响。呈显出白云岩化程度越高,铁和锰含量也越高,白云岩化程度越高,其碳、氧同位素值越向负值方向偏移的特征。从岩石学和地球化学指标来看,白云岩化过程是蒸发泵、渗透回流和埋藏白云岩化多种作用叠加的结果。

The dolomite in Ma-5_5 submember of the Lower Ordovician Majiagou formation is characterized by large thickness and rapid pin-chout laterally, which is significantly different from the relatively thin dolomite with good lateral continuity in Jingbian gas field. The dolo-mite in Ma-5_5 submember in the study area is dominated by micritic dolomite, crystal powder dolomite and fine-medium dolomite, whose de-gree of order is relative low and proportional to crystal size. The Sr content in micrite（algae） of this horizon is far less than that of the con-temporary micrite globally and the residual Sr content is affected by many factors such as fresh water and dolomitization. It is found that thehigher the dolomitization degree is,the higher the contents of Fe and Mn will be. Carbon and oxygen isotope values are generally negative.The higher the dolomitization degree is,the more negative the carbon-oxygen isotope value tends to be. From the view of petrological andgeochemical indices, the genesis of dolomite should be attributed to the combined action of evaporative pumping,seepage reflux and burialdolomitization.