The United Steelworkers union has reached an agreement with Shell that could soon lead to the end of a six-week strike that has affected 12 U.S. refineries — including two in Louisiana.

The union and Royal Dutch Shell have been negotiating a contract that would serve as a blueprint for agreements with other refiners.

About 800 workers at Motiva refineries in Convent and Norco, co-owned by Shell, and at a Shell Chemical plant in Norco joined the strike in late February. The strike has involved more than 6,500 workers at 12 refineries and three chemical facilities in several states over what the union said are inadequate staffing levels, worker fatigue and safety issues.

The union said Thursday the tentative four-year agreement provides yearly wage increases and maintains the level of health care cost-sharing. The agreement also provides for a review of staffing and workloads at refineries.

Shell did not immediately respond to a request for comment. One of the refiners represented by Shell, BP PLC, declined to comment.

The union had accused refiners of creating unsafe conditions by relying heavily on overtime work and using a growing number of contract workers, who the union says aren’t as well-trained as its members.

Union workers are expected to return to work as their local chapters reach agreements with refineries.

Wayne Ranick, a spokesman for the union, said the strike would continue until contracts are ratified.

Murray Pellegrin, a spokesman for Local 13-750 at the Motiva Convent refinery, said Thursday afternoon he was on the road and had not heard details of the tentative agreement. Pellegrin said information about the deal was supposed to be given out to union members during a meeting in Lutcher Thursday night.

A ratified agreement would end the first nationwide refinery strike in more than 30 years.

The strike began Feb. 1 with 3,800 employees at nine refineries, mostly in Texas and California, and later spread to other plants. Companies continued to operate most of the refineries with management and contract employees, although Tesoro Corp. shut down a plant in Martinez, California, that had been partially idled for maintenance.

The union represents workers at about 65 U.S. refineries.

The union said the agreement accomplishes the major goals directed by the union’s oil conference in October and has been approved by the union’s lead negotiators and National Oil Bargaining Policy Committee.

“We salute the solidarity exhibited by our membership,” said United Steelworkers International President Leo W. Gerard. “There was no way we would have won vast improvements in safety and staffing without it.”

Safety issues were central to the negotiations, and the proposed agreement calls for the immediate review of staffing and workload assessments, with union safety personnel involved at every facility. Daily maintenance and repair work in the plants was another critical issue that was addressed, the union said.

“The new agreement calls for joint review on the local level of future, craft worker staffing needs,” said United Steelworkers International Vice President Tom Conway. “Included are hiring plans to be developed in conjunction with recruitment and training programs.”

The next step in the bargaining process is for Royal Dutch Shell to put the terms of the settlement agreement on all of the Shell and Motiva bargaining tables. The union said it expects other employers will offer the same terms at their local bargaining tables.

United Steelworkers represents 850,000 workers in North America employed among industries that include metals, rubber, chemicals, paper and oil refining, plus the service and public sectors.