Digboi. Crude oil was discovered here in late 19th century and first oil well was dug in 1866. Digboi is known as the Oil City of Assam where the first oil well in Asia was drilled. The first refinery was started here as early as 1901. The colonial era saw the first oilfield being developed by the Assam Oil Company, a private entity, in Digboi in Tinsukia district of Assam, where oil was discovered barely eight years after Edwin L Drake drilled the world's first oil well in 1859 in Titusville, Pennsylvania, US.