The dolomitic rocks of the Permian Fengcheng Formation are the most significant hydrocarbon reservoir rocks in Wuerhe—Xiazijie area in the Junggar Basin. About a hundred milliontons of oil (3P) reserves have been found. On the basis of predecessor's studies, unifying petrology and geochemistry methods, this article explores origin of the dolomitic rocks, and provides a scientific geological foundation for exploration and evaluation of this tight oil reservoir in the northwestern margin of the Junggar Basin. By X ray diffraction, cathodoluminescence and scanning electron microscope, and microscopic identification of rock thin sections, we concluded that the dolomitizing host rock consists of tephra and tuffaceous debris of terrigenous origin. The obvious residual texture of replacement in the dolomite minerals under microscope indicates that the dolomitic rocks in the area had not been deposited initially. Strata thickness and mineral assemblage both are the important evidences that reject the viewpoint of hydrothermal jet. Element analyses indicate that, with intensified dolomitization, rock chemical compositions had regularlychanged. The ratios of Yb/La and Yb/Ca of special rocks fall within the scope of carbonatite, indicating that the dolomitizing initial rock has obvious magmatogene.Comprehensive analysis shows that, the dolomitizing initial materials are volcanic ashes. After sedimentation, alteration and calcilization, the volcanic ashes were unequally replaced by Mg2+.