Natural gas appears underground often under great pressures. If the earth
were old, the gas would have seeped out and the reservoirs would have
emptied.
As with oil, the high pressures show that rocks
trapping the
gas are impermeable enough to hold the reservoirs for many millions of
years. If the assumptions (young earth and leaky rocks) behind the
claim were true, the pressures never would have built up in the first
place.
It is also important to remember that pressure does not mean much to
seepage rate without a pressure gradient. If the pressure around a
reservoir of gas is as great as the pressure within it, the gas will
not be forced out.
A geological event that could cause oil and gas to migrate into a
reservoir could have occurred relatively recently. Even if the gas
reservoir is young, that does not mean the earth is young.
