* Refining operations at Cartagena unaffected
* Expansion project will more than double capacity
* Dispute is over pay
BOGOTA, Sept 20 Construction workers expanding Colombia's biggest oil refinery went on strike on Friday in a dispute over a pay, halting the $6.47 billion project but leaving refining operations unaffected.
The stoppage is the first in the country's oil sector in nine years, though labor disruption has been widespread this year in the country's coal mining sector, with weeks-long stoppages at the two biggest producers of the mineral.
U.S.-listed CB&I, or Chicago Bridge and Iron Co, has been tasked with more than doubling capacity at the Cartagena refinery known as Reficar, to 165,000 barrels by 2015 from 80,000 at present.
Reficar, fully owned by state-run oil producer Ecopetrol , said its refining operations were unaffected by the stoppage of roughly 10,000 workers on the project, which began at 10 a.m. on Friday.
"We started a strike over the intransigent policies of CB&I with regard to demands of workers who earn salaries well below those paid by other companies in the sector," said Rodolfo Vecino, president of the USO, an oil sector labor union.
While the USO union says it wants wage increases of up to 43 percent depending on job function, CB&I says it has offered increases of between 23 percent and more than 100 percent of workers' incomes and benefits.
"The company laments the start of the strike by the union and its consequences, particularly the suspension of labor contracts of 10,000 workers of CB&I Colombiana SA who will no longer receive their salaries," Reficar said.
CB&I and the union were continuing negotiations to reach a deal enabling them to suspend the strike and for workers to return to their jobs, the union and Reficar said.
The expansion project at Reficar, which focuses on serving the export market, is vital, so that Ecopetrol can increase refining capacity to 650,000 barrels per day by 2015.
Colombia's biggest refinery, Barrancabermeja, is in the central province of Santander and has a refining capacity of 250,000 barrels of oil daily.