UPDATE 1-Marathon's La. expansion online by end 2009-exec

(Updates with project details, background throughout)

HOUSTON Oct 29 (Reuters) - Marathon Oil Corp (MRO.N) is starting up units in a 180,000-barrel-per-day (bpd) expansion of its Garyville, Louisiana, refinery and expects the added capacity to be online by a year-end deadline, the company's refining chief said on Thursday.

"We are in the sequence of bringing up our brand-new refinery in Garyville," said Gary Heminger, president of Marathon Petroleum Co, Marathon Oil's refining subsidiary, during a speech at a Houston refining conference. "We expect to have that up by the end of the year."

Since starting the project in the wake of 2005's Hurricane Katrina, Marathon has said the expansion was essentially the construction of a new refinery next door to the existing plant located west of New Orleans.

The Garyville expansion start-up comes at the same time an estimated 750,000 bpd in U.S. refining capacity has been turned off as the economic downturn crushed demand for motor fuels across the country.

The $3.2-billion expansion will bring the refinery's total capacity to 436,000 bpd. (Reporting by Erwin Seba; Editing by Christian Wiessner)