 Discovery of oil near Kilgore, Texas, in October 1920 was the beginning of an oil boom that attracted workers to the county and expanded the population by more than 500% by 1940, according to the census.By that time, the economy had stabilized but the East Texas Oil Field, extending in five counties, has continued to be important to the county and region's economy.This area was among early sections settled by United States immigrants before Texas became an independent republic, and after 1845, a state of the United States.