Negotiators for the United Steelworkers Union and Shell Oil Co. announced they have reached an agreement on a new contract, after a month-long strike at 15 oil refineries, including two in California.

The previous contract, which covered 30,000 workers expired Jan. 31. Work stoppages, affecting 6,500 employees, began the next day.

Refineries in Carson and Martinez, owned by Tesoro Corp., were among the targeted facilities. Tesoro chose to shut down the Martinez plant, which was already in limited operation because of maintenance.

The strike, impacting refineries responsible for 20% of the nation’s capacity, was the largest in 35 years.