Abstract: Based on the analyses of carbon and oxygen isotope composition and palynofacies of carbonate source rocks (P₁q-T₁q) from the South Yellow Sea Basin,the formation and controlling factors of effective source rocks of marine facies were studied.The correlation between δ¹³C and δ¹⁸O indicated that the carbon and oxygen carbon isotope compositions were similar to those in original ocean.The δ¹³C_PDB values in P₁q were heavier than those in T₁q,indicating for higher biological productivity,which was favorable for source rock generation.Based on mean value,the δ¹³C_PDB values of P₁q were divided into 3 evolution stages corresponding to the changes of palynofacies,indicating that the formation of source rocks varied among different layers.The δ¹³C_PDB values in P₁q from well WX5-ST1 were 3.83‰ in average,obviously heavier than those from well CZ24-1-1 (-1.34‰ in average).It corresponded to the high content of amorphous organic matter,and proved the differences of source rock formation among different regions in the same layer.The comparison of carbon and oxygen isotopes and palynofacies showed that the effective matching of biological productivity and preservation condition was favorable for source rock formation.