To what extent must the oxygen content be reduced before an explosion of methane and air is impossible?
For a mixture of strictly methane and air, the oxygen content must be 12 percent or less. 
It is however, significant to note that there is a combination of combustible gases following explosions and during fires.
So the maximum allowable oxygen will vary according to variations in the combustible and inert gases.
12 percent
10 percent
11 percent
7 percent