El Dorado is a city in and the county seat of Union County, Arkansas, USA. According to estimates, the 2012 census, the population of the city is 18,491, which represents a decline of 2.1 percent from the 2010 tabulation of 18,884. El Dorado is headquarters of the Arkansas Oil and Gas Commission as well as Murphy Oil, Murphy USA, Deltic Timber Corporation, and the Lion Oil Refinery.