The 20 Biggest Oil Discoveries. This map includes 1,232 discoveries of recoverable reserves over 500 million barrels of oil equivalent (BOE) From 1868 to 2010. The discoveries cluster in certain parts of the world, covering 46 countries, and are of significant magnitude for each country’s economy. An oil refinery complex is where crude oil is treated into different products such as diesel, kerosene or gasoline. What is meant by oil producing country is how many crude oil is coming out of the country's wells. For instance India has one of the biggest refinery in term of capacity but it's not even in top 20 of oil producing countries.