Oil took millions of years to form and the oil supplies in the ground won't last for ever.
The oil fields already discovered hold over 1 million million barrels of oil (1,000,000,000,000).
Although we are using oil quite quickly, the reserves go up every year. This is because more oil is discovered and new ways are found of extracting oil that couldn't be got out before.
Even so, our oil won't last for ever. At the moment, the world uses about 26,000,000,000 barrels every year. At this rate, there should be enough oil for at least another 40 years.
It is likely that more oil will be discovered in that time.
Oil companies are always searching for new oil fields and there are still lots more deep sea areas to explore.