The SAGD process of heavy oil or bitumen production is an enhancement on the steam injection techniques originally developed to produce heavy oil from the Kern River Oil Field of California. The key to all steam flooding processes is to deliver heat to the producing formation to reduce the viscosity of the heavy oil and enable it to move toward the producing well. The cyclic steam stimulation (CSS) process developed for the California heavy oil fields was able to produce oil from some portions of the Alberta oil sands, such as the Cold Lake oil sands, but did not work as well to produce bitumen from heavier and deeper deposits in the Athabasca oil sands and Peace River oil sands, where the majority of Alberta's oil sands reserves lie. To produce these much larger reserves, the SAGD process was developed, primarily by Dr. Roger Butler of Imperial Oil with the assistance of the Alberta Oil Sands Technology and Research Authority and industry partners. The SAGD process is estimated by the National Energy Board to be economic when oil prices are at least US$30 to $35 per barrel.