 Prudhoe Bay Oil Field on Alaska's North Slope is the largest oil field in North America, The field was discovered on March 12, 1968, by Atlantic Richfield Company (ARCO) and is operated by BP; partners are ExxonMobil and ConocoPhillips Alaska.In September 2012 Shell delayed actual oil drilling in the Chukchi until the following summer due to heavier-than-normal ice and the Arctic Challenger, an oil-spill response vessel, not being ready on time.However, on September 23, Shell began drilling a "top-hole" over its Burger prospect in the Chukchi.