ORIGIN OF DOLOMITE OF ORDOVICIAN MAJIAGOU FORMATION, WESTERN AND SOUTHERN MARGIN OF THE ORDOS BASIN

A large amount of dolomite is developed in the Ordovician Majiagou Formation in western and southern margin of the Ordos Basin.The massive dolomite which has large exploration potential mainly distributes in the northern Tianhuan Depression and Weibei Uplift.Based on the study of outcrop section and borehole core,the lithology and mineralogy characteristics,the geochemical data and sedimentary diagenetic-structural setting,the dolomite in study area can be divided into three types: penecontemporaneous dolomite,burial dolomite and hydrothermal dolomite.Penecontemporaneous dolomite developed in Majiagou Formation Member 1,3 and 5.Its origin is evaporative pumping dolomitization.The genesis of burial dolomite which forms main body of the dolomite of Majiagou Formation can be explained as seepage-reflux dolomitization during shallow-burial stage and compacting flow dolomitization and landform-driven flow dolomitization.Hydrothermal dolomite which fills in fractures is discovered in Dingbian,Fuxian,Huangling and Linyou area.The deep thermal fluid rising along faults and fracture systerm is relate to the origin of hydrothermal dolomite,it plays an important role in forming reservoirs in this area.In addition,this article describes the textural characteristic and evolution of the dolomite,for having an intuitive cognition with the forming process of various types of dolomite.