Direct injection (DI) natural gas engines are modern engine concepts providing clean combustion and high fuel efficiency. Even at high injection pressures, such engines operate at heterogeneous/stratified fuel/air mixture conditions due to the relatively short mixing time of the fuel jet. The sub-optimal fuel-air mixture results in emissions of unburnt hydrocarbon (UHC). In particular, one of the most severe UHC emission is the release of the unburnt greenhouse gas methane (CH4) into the atmosphere (methane slip). To better understand the origin of methane slip, in-depth knowledge of the turbulent mixture formation process is required. It is therefore critical to model the turbulent fuel jet using state-of-the-art computational fluid dynamics (CFD) methods with high time and space accuracy. In the present study, the penetration and mixing of non-reacting methane and nitrogen jets are simulated and compared. Nozzle pressure ratios between 4.5 and 10.5 are investigated with respective Reynolds numbers of the order 100,000. Based on these results, novel information is provided in terms of: (1) demonstration of the influence of the fuel molecular mass, and the injection pressure on turbulent mixture formation in highly underexpanded jets, and (2) understanding of the fuel air mixing dynamics for transient injection. The results indicate that, typically, the CH4 jet mixes faster than the heavier N-2 jet. Investigation of the average density distribution explains the mixing differences. (C) 2014 Elsevier Ltd. All rights reserved.