HOUSTON, May 23 (Reuters) - Saudi Aramco plans an investment of up to $30 billion in its U.S. subsidiary Motiva Enterprsies LLC, the company said in an announcement at a business summit in Saudi Arabia.

The company said that $12 billion would be the initial investment in a project to expand refining capacity at Motiva's Port Arthur, Texas, refinery, already the largest in the United States, and to extend Motiva's operations in the petrochemical value chain, according to a statement about the investment.

A likely additional investment of $18 billion is expected in Motiva by 2023, it said.

Since the completion of an expansion of the Port Arthur refinery in 2012, which more than doubled its capacity to refine crude oil to 603,000 barrels per day (bpd), Motiva has weighed plans for further expansion of the plant.

Saudi Aramco has also looked at acquiring at least one additional Gulf Coast refinery and visited chemical plants up for sale.

"We are investing in long-term job creation and the future of the refining industry in the United States, and we are delivering on Vision 2030 to expand the U.S.-Saudi partnership," Saudi Aramco President and Chief Executive Amin Al Nasser said on Saturday, according to the statement.

Aramco said in the announcement that in the short-term an additional 2,500 jobs would be created in Port Arthur and an additional 12,000 jobs by 2023.

Vision 2030 is a plan announced last year by Saudi Deputy Crown Prince Mohammed bin Salman to diversify the Saudi economy away from reliance on oil production.

On May 1, Motiva became an Aramco subsidiary with the breakup of a 19-year partnership with Shell. Under the breakup agreement, Aramco retained the Motiva name and the Port Arthur refinery along with distribution operations across seven states.

(Reporting by Erwin Seba; Editing by Tom Brown) ((erwin.seba@thomsonreuters.com; +1-713-210-8508; Reuters Messaging: erwin.seba.thomsonreuters.com@reuters.net))