Crude oil reservoirs and major producersCrude oil is formed over millions of years by organic materials decomposing in a low oxygen, high pressure environment. Due to the time it takes for crude oil to form, it is considered a fossil fuel, and its reserves are finite. As an increasing number of conventional reserves such as underground reservoirs were depleted, unconventional means of oil extraction have become more viable. Through their exploration of shale oil and oil sands, the United States and Canada have become major oil-producing countries. Over four billion metric tons of oil is produced worldwide every year, with the Middle East housing the greatest share of proved oil reserves, at close to 50 percent.
With oil serving such a great number of purposes, it is unsurprising that oil (and gas) companies are among the largest corporations worldwide. In 2021, Chinese state-owned China National Petroleum Corporation (CNPC) and the Sinopec Group ranked fourth and fifth, respectively in a list of the largest company in the world based on revenue. Although many oil companies had reported financial struggles throughout 2020 due to the coronavirus pandemic, three other oil majors were also ranked among the 20 largest companies.