A hydrocarbon reservoir is a geological formation that contains accumulations of hydrocarbons, which are organic compounds made primarily of carbon and hydrogen. These reservoirs can be found in various rock types, such as sandstone or limestone, and are often located deep beneath the Earth's surface. The hydrocarbons can exist in liquid form, known as crude oil, or as gas, referred to as natural gas. The formation of hydrocarbon reservoirs typically involves the burial of organic material, which, over millions of years, undergoes heat and pressure to transform into hydrocarbons. These reservoirs are crucial for energy production, as they serve as the primary source for extracting fossil fuels used in various applications, including transportation and electricity generation.