ExxonMobil is considering a multi-billion-dollar expansion of its Beaumont refinery in Texas, according to a weekend report from news agency Reuters.

The expansion would be among the first major refining investments of the US shale boom.

If carried out, the expansion would potentially more than double the size of the 344,600 bpd Beaumont refinery with the addition of a third crude distillation unit (CDU), Reuters reported.

An Exxon spokesman declined to discuss plans for the Beaumont refinery and said the company was always evaluating growth options.

The largest US refinery at present is Motiva's 600,000 bpd refinery in nearby Port Arthur, Texas. However, ExxonMobil is reportedly looking at making the Beaumont plant even bigger.

"They've talked between 700,000 and 800,000 bpd in total refinery capacity," a source told Reuters.

More modest near-term projects to upgrade and expand coking units to refine more heavy crudes are already underway, according to the report, which cited sources saying Exxon was moving forward on plans to replace four coking unit drums in 2015 and add two new coker drums in 2017.