Abstract: There is a close relation between the porosity and vitrinite reflectance(R_o)of kerogen in the mudstones of Bayanhua Group,lower Cretaceous in Hongholshut depression.By a correlation,it is found thatthe relation between the porosity and vitrinite reflectance is most closely approximating to a power function.Based on this power functional relation,porosity was converted from the seismic velocity of mudstones,then the vitrinite reflectance was calculated from the porosity,finally to delineate the immatureand mature areas for the main hydrocarbon source rocks in the sag,and provide evidences for the selectionof near-source targets.The method that measures organic thermal maturity directly from seismic information is of great value for areas with lower exploratory levels,especially for the basins with sparse welldrillings.