With the analysis of the distribution，lithology，color and mineral features under SEM of the dolomite，the origin of the dolomite in Block1 in Yemen and the impact of dolomitization on oil and gas accumulation were studied. The thickness of dolomite in Block1 is large but it distributes in a very limited area near Well Imad-1. The crystals of dolomite look like sugar with main features of saddle dolomite. Only the traces of shell fossils are observed. There is no sphere，irregular thorns and rhombic shapes observed in dolomite cores and thin sections. And the dolomite is grey-dark grey，the dark color of which indicates that the dolomite contains high contents of ferrum and manganese. All these feature points to hydrothermal genesis of the dolomite of Block1 in Yemen. It is inferred that hydrothermal fluid comes mainly from the deep area with abundant Mg2+ ions. It flowed upward to the upper strata through the faults or disconformity and exchange between Ca2+ from the limestone and anhydrite and Mg2+ from the hydrothermal fluid occurred. Finally，some thick dolomite rocks developed near the faults（or disconformity）with limited lateral distribution. The formation of dolomite can increase the pore spaces of the reservoir，but it can damage the cap rock and preservation condition. Considering the influence of faults as well，it is believed the southeast area in Block1 has lower potential of oil and gas accumulation，but the Magraf belt in northwest area is suggested as future exploration target in Block1.