HOUSTON (Reuters) -Marathon Petroleum continued assessing damage on Friday to the shut residual hydrotreating unit (RHU) at its 631,000-barrel-per-day Galveston Bay Refinery in Texas City, Texas, from a June 14 fire, said people familiar with plant operations.

The assessment began after the fire on the 64,000-bpd RHU was extinguished on the night of June 14, the sources said. How long the unit may remain shut was unknown.

Marathon spokesperson Jamal Kheiry declined to comment on the downed unit on Friday.

Production was cut back on the gasoline-producing 140,000 bpd fluidic catalytic cracker 3 (FCC-3) and the 115,000 cat feed hydrotreater due to the fire, the sources said.

No injuries were reported from the fire that broke out at about noon on June 14.

Residual crude oil is the gunky, tar-like oil left after completion of multiple steps in the process of refining crude oil into gasoline, diesel and jet fuel.

The RHU uses hydrogen to extract motor fuel feedstocks from the residual crude and remove sulfur in compliance with U.S. environmental rules.

The Galveston Bay Refinery is the second largest in the United States by capacity, according to a report issued on Friday by the U.S. Energy Information Administration.

Texas City is located 42 miles (68 km) southeast of Houston.