Oil and gas are less dense than water, so they migrate through porous sedimentary rock toward the earth's surface. When the hydrocarbons are trapped beneath less-porous cap rock, an oil and gas reservoir is formed. These reservoirs of oil and gas represent our sources of crude oil and natural gas.

The upstream segment is characterized by high risks, high investment capital, extended duration as it takes time to locate and drill, as well as being technologically intensive. Virtually all cash flow and income statement line items of E&P companies are directly related to oil and gas production.