Giant oil company Chevron Corp plans to restructure its refinery operations, cutting some jobs and exiting some markets, a company spokesman said on Tuesday.

The company, the fifth-largest refiner in the United States, issued a memo to employees on Monday laying out the broad outlines of the restructuring, which is expected to be completed in the third quarter.

It's going to be a leaner organization with fewer positions, but we have not yet determined the number of positions we need, Chevron spokesman Lloyd Avram said.

The company will exit from certain markets, he said, although which markets the company will leave has not yet been determined.

Chevron has also not decided whether to close any of its refineries, he said. Further details are expected to be released in March.

Earlier this month, Chevron warned that its fourth-quarter results would be sharply lower than those of the previous quarter due to a further deterioration in refining margins.

Shares in Chevron dipped 0.5 percent to $78.88 in early trade on the New York Stock Exchange.

(Reporting by Matt Daily, editing by Gerald E. McCormick)