The Alaska North Slope region includes the National Petroleum Reserve-Alaska, with the bulk of Alaska's known petroleum until the Prudhoe Bay Oil Field was discovered in 1968, followed by the Kuparuk River oil field in 1969. The region also includes the Arctic National Wildlife Refuge, which itself has been the subject of controversy surrounding the possibility of petroleum drilling within its boundaries. The petroleum extracted from the region is transferred south by means of the Trans-Alaska Pipeline System to Valdez on the Pacific Ocean. Under the North Slope is an ancient seabed, which is the source of the oil. Within the North Slope, there is a geological feature called the Barrow Arch - a belt of the kind of rock known to be able to serve as a trap for oil. It runs from the city of Utqiagvik to a point just west of the Arctic National Wildlife Refuge. The Arctic Alaska Petroleum Province, encompassing all the lands and adjacent Continental Shelf areas north of the Brooks Range-Herald arch (see map) were estimated by the USGS in 2005 to hold more than 50 billion bbl of oil and natural-gas liquids and 227 trillion cubic feet of gas.