The second stage of the 40,000-bpd hydrocracker was taken out of production on June 8 because of a malfunction, the sources said.
Shell did not reply to a request for comment.
The first stage of the hydrocracker continued to operate while the second stage was shut, according to the sources.
Hydrocrackers convert gas oil or other feedstocks into diesel and other motor fuels by using a catalyst in the presence of hydrogen under high heat and pressure.
(Reporting by Erwin Seba; editing by Jonathan Oatis)