Strike at Marathon's Galveston Bay plant to continue -USW official

(Adds activity at other refineries, details, background)

HOUSTON, March 13 (Reuters) - A strike by the United Steelworkers (USW) local union at the Marathon Petroleum Corp Galveston Bay Refinery in Texas City, Texas, will continue as long as the company refuses to accept the terms of a newly-negotiated national agreement without encumberances, said a union official on Friday.

The national agreement, often referred to as "the pattern agreement," reached on Thursday by officials from the union and lead refinery owner negotiator Shell Oil Co is supposed to be offered by the companies at all refineries and chemical plants represented by the USW.

"The company refuses to offer the pattern unencumbered," said W.E. Sanders, sub-director for USW District 13, which covers Arkansas, Louisiana, Oklahoma and Texas. "The company clearly knows its obligations under the pattern."

A Marathon representative was not immediately available to discuss the dispute.

Marathon is the only company so far said to be unwilling to offer the national agreement at one of its locations. Six companies own the 15 plants where 6,550 USW members remain on strike pending formal ratification of the national and local agreements, which will include the terms of return-to-work plans.

Striking workers are expected to return to their jobs within two weeks.

Workers at Marathon's Catlettsburg, Kentucky, refinery were expected to remain off of their jobs while the company and the USW discuss local issues, according to a report from the Huntington, West Virginia, Herald-Dispatch.

Shell (LSE: RDSB.L - news) , which is the U.S. arm of Royal Dutch Shell Plc (Xetra: R6C1.DE - news) , and the USW were negotiating on Friday a return-to-work agreement framework that will be used by the local unions.

The strike by the USW began on Feb. 1 and spread to 15 plants, including 12 refineries that account for one-fifth of U.S. capacity. It is the largest national work stoppage by refinery workers since 1980.

Workers at the Shell refinery and chemical plant in Deer Park, Texas, and three Motiva Enterprises refineries, of which Shell is a co-owner, were awaiting the return-to-work framework agreement, expected by Saturday.

Negotiations were continuing between the USW local union and Lyondell Basell Industries over local issues at the company's Houston Refinery, said a source familiar with the talks. A federal mediator has joined the talks.

A federal mediator may also join the talks between Marathon Galveston Bay workers and the company when they restart, Sanders said. (Reporting by Erwin Seba; Editing by Diane Craft, Bernard Orr)