Coexistence of sI and sII in methane-propane hydrate former systems at high pressures

•New equilibrium data up to 100 MPa are presented for CH4-C3H8 gas hydrates.•Influence of experimental factors on HP-µDSC and isochoric results were evaluated.•Both sI and sII are found in CH4-C3H8 hydrate-former systems above 10 MPa.•Fractions of sI and sII depend on gas phase compositions and the methodology applied.•Mixing and high subcooling may favor the methane hydrate (sI) formation.