HOUSTON, June 21 (Reuters) - A fire was extinguished in a furnace at the co-generation plant providing steam and power to Marathon Petroleum's MPC.N 631,000 barrel-per-day (bpd) Galveston Bay Refinery in Texas City, Texas, on Sunday, according to two people familiar with plant operations.

Marathon spokesman Jamal Kheiry did not identify the unit where the fire broke out at the Galveston Bay Refinery, which is the nation's second largest. Refinery firefighters "extinguished a small fire," Kheiry said. No injuries were reported.