Most of the Earth's oil is found in sedimentary basins. These are large, bowl-shaped areas where layers of sediment, like sand, silt, and mud, have accumulated over millions of years.

Here's a breakdown:

* Middle East: This region holds the largest proven oil reserves in the world, accounting for roughly 48% of the global total. Key countries include Saudi Arabia, Iran, Iraq, Kuwait, and the United Arab Emirates.

* North America: The United States, Canada, and Mexico are significant oil producers, with the majority of their reserves located in sedimentary basins along the Gulf of Mexico and the Western Interior Seaway.

* South America: Venezuela, Brazil, and Colombia have significant oil reserves, mostly found in the Orinoco Belt and the Amazon Basin.

* Russia: This country holds the world's largest natural gas reserves and is also a major oil producer, with significant reserves in Siberia and the Volga-Ural Basin.

* Other regions: Other countries with significant oil reserves include China, Libya, Nigeria, and Kazakhstan.

It's important to note that oil reserves are constantly changing as new discoveries are made and existing reserves are depleted.