With the ice-free drilling season nearing an end, Shell Oil started its first exploration well in the Chukchi Sea off the northwest coast of Alaska on Sunday.

At 8:30 a.m. Eastern, the Noble Discoverer began drilling. On Friday, the drilling rig was moored to eight anchors, spread in a circular pattern 6,500 feet across on the sea floor to center the rig over the well. Each anchor weighs several tons.

The company said crews on board would drill a pilot hole roughly 1,400 feet deep, stopping well short of the oil-bearing reservoirs while the company finishes work on a spill- control vessel it needs in place before getting final permits.

Last week, the Interior Department said Shell could drill the pilot hole and the 20-by-40-foot hole called the mud line cellar, which will house the blowout preventer and allow it to sit below the sea floor. The company also will install casing and cement in the top portion of the well.

But Shell is supposed to stop drilling Sept. 24 in the Chukchi, and it might end up sealing its partially completed wells and returning to finish drilling next summer.

Shell has spent nearly seven years and more than $4 billion trying to get an exploration well drilled in the Chukchi, which the company believes could hold enormous oil reserves.

But its drilling plans have been the center of a politically charged battle over whether oil exploration in the Chukchi and Beaufort seas would disturb whales, walruses and seals, and whether Shell would be able to control an Arctic oil spill in the unlikely event one were to happen there. Many Native Americans who live along Alaska’s Arctic Coast also are worried about the possible threat drilling might pose to their subsistence hunting.

Weather at the drill site was overcast Friday. Shell has six support vessels, including one for breaking ice and another for spill response. The company also has Alaska native protected species observers on every vessel and representatives from the Bureau of Safety and Environmental Enforcement on board the drill ship.

Shell, part of Royal Dutch Shell, had been planning to start drilling earlier, but was delayed by an unusually heavy ice season in the area of its well and by its inability to finish fixing deficiencies in its Arctic Challenger barge. Interior Secretary Ken Salazar said recently that Shell would get permission to drill into oil-bearing reservoirs only when the Arctic Challenger meets federal standards.