—there are no doubt many details of which we are unaware, of processes below about 25000 feet in the crust of the earth-—
Oil is not usually found below 10,000’ except in “cool” basins, such as the Gulf of Mexico. At higher temperatures, the Earth acts like a refinery and cooks the oil into more volatile members, like methane, butane, etc... Very little natural gas has been found below 20,000’ (except in these “cool” basins) because the overburden of the Earth crushed the porosity so that there is no place for the gas to reside.