Shell spokesman Ray Fisher said planned maintenance was continuing at the Norco refinery but did not identify the units involved.

Production on the refinery's 240,000 bpd DU-5 crude distillation unit (CDU) is cut back while hard carbon deposits were being cleaned from the coker, the sources said.

The coker was shut on Jan. 10 for the work.

Cokers convert residual crude oil from distillation units into either hydrocarbon feedstocks for motor fuels or petroleum coke, a coal substitute.