Why crude oil only found in certain parts of the world?

The simple answer is oil, like coal, was formed from ancient forrests that laid down millions of years of vegetation that for some reason (flooding, climate change, continental drift) stopped growing and were trapped into pockets that were covered by layers of rock and eventually pressed into coal fields and oil fields. Its not found everywhere because all the conditions needed for the process to work didn't exist everywhere. Oil is an interesting case. No one knows for sure how exactly it was created. You don't find vegetation fossils in oil like you do in coal beds and oil contains Helium, which is not used by plants. Physicist Tommy Gold (recently died) theorized that oil started as Methane (natural gas) ,which is the most common compound found in the universe, trapped in the earth when it was formed. The gas perculated up into a sub layer, like the water table, and was processed (eaten and crapped out) by microbes into oil. This sublayer of oil (estimated to be about 60 miles down) finds cracks above it, leading to pockets closer to the Earth's surface and slowly percolates up into these pockets, which we discover as oil fields. One way he suggested to test his theory was to revisit old pumped out oil fields and see if they had partially filled back up.