Natural gas is higher temperature. Unlike coal, which is terrestrial , coming from swamps, oil is from continental shelves and restricted basins in the Ocean. They require that they form in a permeable rock. They float up and must find a trap cap rock. Mesozoic and Cenozoic in Age.
Older oil is probably limited because it has already escaped.
Tar sands and oil shales and other non-permeable deposits will be explored in the future. They are too solid. We re talking big money. In contrast to coal, the impact on the environment from exploration and exploitation is minimal. Easy to transport No leftovers .