Abstract: The Pre?Caspian basin is one of the giant oil?gas bearing basins in the world, and the Carboniferous carbonate rocks in its eastern margin are favorable hydrocarbon reservoirs. By means of logging interpretation, core analysis and contrastive analysis, existence of the Дlayer dolomite in KT?Ⅱpetroleum system in southern Zahnanor oilfield was verified for the first time. The study of the dolomitization feature and reservoir control factors show that the dolomite is mainly distributed in the bottom of Д1 layer, and the rock porosity tends to increase with the increase of dolomite content, but the trend is not obvious when dolomite content is less than20%. The dolomitization does not always increase the porosity, only a certain degree of dolomitization can significantly influence on petrophysical property, too low or too high degree of it is unfavorable to improve the reservoir quality. When the dolomite content is over80%, the rock porosity may decrease to about6.5%. The control factors on the carbonate reservoirs in this area are multiple, of which dolomitization is of great importance and crucial for the development of high quality reservoirs in this area.