Nigeria has a total of 159 oil fields and 1,481 wells in operation according to the Department of Petroleum Resources. The most productive region of the nation is the coastal Niger Delta Basin in the Niger Delta or "south-south" region which encompasses 78 of the 159 oil fields. Most of Nigeria's oil fields are small and scattered, and as of 1990, these small fields accounted for 62.1% of all Nigerian production. This contrasts with the sixteen largest fields which produced 37.9% of Nigeria's petroleum at that time.