Shell announced its drilling rig Kulluk left Dutch Harbor, Alaska, for prospective drilling sites in arctic waters.
Regional conditions mean Shell needs to finish work in the Chukchi and Beaufort seas by October to avoid reforming ice sheets.
"At this time, ice is still present in both the Chukchi and Beaufort seas, although no longer at the prospect sites," the company said in a statement.
This late in the season, however, Shell said its drilling program would be diminished.
Environmentalist groups expressed concern about the potential for damage to the arctic should a spill take place. U.S. Interior Secretary Ken Salazar said last week he'd hold Shell's "feet to the fire" to ensure the company meets federal regulations in the region.
Shell said it's dedicated to meeting federal safety regulations regarding its oil spill response plans.
The company aims to access an offshore region said to have more than 20 million barrels of oil.