genesis of gas at these depths, page-11

There are a couple of misconceptions I thought I'd try to clear up.
Firstly gas and oil are effectively cooked out from high organic content shales deep in the basin center where temperature and pressure are high. Over millions of years the hydrocarbons migrate into traps.
To have a petroleum accumulation you need 1) a seal, 2) a reservoir, 3) a structure (simplistically a sandstone dome covered by a seal, like salt or claystone) and 4) charge, which implies source rocks exist in the basin and have been buried deeply enough to expunge oil or gas.
If one of the above criteria is defective then the oil and gas will either not accumulate or leak away.

So in summary, the gas is not created in the reservoir, it migrates into it usually a long time after the reservoir, seal and structure have been created.