The Chicontepec Basin is a petroleum system in Mexico north-east of Mexico City, covering an area of around 3,800 square kilometres (1,500 sq mi) in the states of Veracruz, Puebla and Hidalgo. Several oil fields have been discovered in that area since 1926. A major field was discovered in 1973 but contains extra heavy crude[dubious - discuss], which due to special refining needs is considered unviable at this time. Since that time, tiny amounts of oil have been produced. While the reserves are overwhelmingly extra heavy crude, pockets of lighter tight oil, as well as natural gas, are also found in the basin.