Shell spokesman Ray Fisher said a unit malfunction triggered flaring at the 325,700 barrel-per-day (bpd) refinery, but declined to identify the unit involved.

The sources did not identify which CDU malfunctioned. The refinery has two CDUs, one with a capacity of 70,000 bpd and the other with a 270,000 bpd capacity.

The CDUs do the primary refining of crude oil and provide hydrocarbon feedstock for all other units.

The Deer Park refinery is a 50-50 joint venture between Shell and Petroleos Mexicanos (Pemex), Mexico's state oil company.