The alkane with the shortest chainlength is methane (CH4), ethane (C2H6) is second etc.

NameFormula
MethaneCH4
EthaneC2H6
PropaneC3H8
ButaneC4H10
PentaneC5H12
HexaneC6H14
HeptaneC7H16
OctaneC8H18
NonaneC9H20
DecaneC10H22
DodecaneC12H26
EicosaneC20H42

Natural gas is largely made up of methane.