The reason that fossil fuel deposits are able to stay stable for millions without spontaneously oxidizing is that there is virtually no oxygen in them. Indeed that oxygen poor environ was the reason they formed fossil fuels in the first place. In an environment with any oxygen, plant material would be metabolized into C02(i.e. rotted) by bacteria and such.
In order for fire to occur, three components are needed: heat, oxygen, and fuel. Remove any one and the fire goes out. This is called the fire triangle.
Because fossil fuel deposits in situ lack oxygen, they are not inflammable except where they escape the deposit and make contact with oxygen. Thus a natural gas deposit can't explode. If someone pokes a hole into it and releases it into the oxygen rich atmosphere, only then it can it explode.