[philipkglass]

LNG is 85-95% methane, typically, with ethane making up most of the rest. It contains small amounts of propane and butane.

I hadn't known that SpaceX was using a special grade of ~100% liquefied CH4 as nickik says above.

EDIT: shale gas is richer in ethane and higher hydrocarbons than traditional sources of natural gas. Maybe SpaceX is sticking to 100% methane just so they don't have to ponder variability of composition depending on how gas is being extracted.

[shmerl]

Methane can be synthesized from CO2 and hydrogen. More complex hydrocarbons are way more difficult to produce. So pure methane based engine is a better method for using on other planets.