We studied formation probability distributions of a model natural gas (a mixture of 90 mol % methane and 10 mol % propane) hydrate in anionic surfactant aqueous solutions under isobaric conditions. The surfactant used in this work was sodium dodecyl sulfate (SDS), which had been reported to promote gas hydrate formation. The range of the mixed gas pressure studied was between 0.1 and 12 MPa, and the range of the SDS concentration studied was between 1 and 100 mM, which covered both above and below the critical micelle concentration (cmc). The results show that SDS inhibited formation of gas hydrates at low concentrations. SDS promoted the formation of gas hydrates only at the highest concentration and at the highest gas pressures studied. The width of the formation probability distributions decreased with increasing the SDS concentration. These results suggest that SDS does not promote nucleation of natural gas hydrates and that the promotion effect of SDS is limited to gas supersaturation, gas hydrate growth rate, and/or final guest occupation ratio of gas hydrates.