HOUSTON (Reuters) -Mexico's national oil company Pemex completed the shutdown of multiple units at its 312,500-barrel-per-day Deer Park, Texas, refinery on Thursday to begin a multi-unit overhaul, people familiar with plant operations said.

The refinery's 270,000-bpd DU-2 crude distillation unit (CDU), the larger of two at the refinery, is shut for the planned overhaul that will last about 60 days, the sources said.

A Pemex spokesperson did not immediately reply to a request for comment.

Pemex calls the overhaul "the Big Block Turnaround" because it involves the large CDU and the 70,000-bpd fluid catalytic cracking unit (FCCU), 70,000-bpd diesel-producing hydrocracking unit (HCU) and 92,000-bpd coker along with other units, sources said.

DU-2 is the larger of two CDUs breaking down crude oil into feedstocks for all other units at the refinery. The small CDU, the 70,000-bpd DU-1, will remain in operation while DU-2 is down for the overhaul.

The FCCU, HCU and coker are all supplied with feedstock by DU-2.

FCCUs use a catalyst under high heat and pressure to convert gas oil into unfinished gasoline.

HCUs use a catalyst under high heat and pressure in the presence of hydrogen to convert gas oil into gasoline.

Cokers break down gunky, tar-like residual crude oil into either motor fuel feedstocks or petroleum coke, which can substitute for coal.