Four petroleum sulfonates including first vacuum distillate sulfonate, second vacuum distillate sulfonate, third vacuum distillate sulfonate, and vacuum residue sulfonate were synthesized. The tensions of the petroleum sulfonates were measured. It shows that the third vacuum distillate sulfonate can produce ultralow interfacial tension when the concentrations are higher than 0.010%. Synergism between Na₂SO₄ and CaCl₂ (or MgCl₂) in the formation water was proved, which is crucial to reduce the tension. Moreover, synergism between the second and third vacuum distillate sulfonates occurs. This work is useful for enhanced oil recovery.