Abstract: An immature type-I mudstone collected from the third section of the Tertiary Shahejie Formation (Es₃³) in the Dongying Sag of the Bohai Bay Basin was studied with laboratory experiments.Products were extracted,fractionated into chemical groups,and re-mixed to obtain synthetic oil (s-oil) with a group composition similar to reservoir oil and a pseudo-kerogen (p-kerogen) composed of 83% kerogen and 17% residual soluble organic matter.The two samples were pyrolyzed in sealed gold tubes under constant pressure and non-isothermal heating conditions and the generated gases were analyzed.The two gases are quite different in their chemical and isotopic composition.Compared with the gas derived from p-kerogen,the s-oil-derived gas is rich in C₂-C₅ hydrocarbons during the early cracking stages and C₁-C₃ hydrocarbons are depleted in δ¹³C throughout the cracking stages.The carbon isotopic ratio differences between the two gases can reach 10‰ (δ¹³ C₁),14‰ (δ¹³ C₂),and 9‰ (δ¹³ C₃).The δ¹³C₂-δ¹³C₃ value of the s-oil gas is much more sensitive to thermal stress than that of the p-kerogen gas and the plots of (δ¹³C₂-δ¹³C₃) versus δ¹³ C₁ and (δ¹³C₂-δ¹³C₃) versus ln(C₂/C₃) are effective in identifying the two gas types.These results provide a guide to differentiate gases derived from oil cracking from those derived from kerogen maturation.