PORT ARTHUR, Tex. — It is hard to imagine the desert sands of the Persian Gulf being any farther away than from this swampy refinery port known for Cajun food, sport fishing and being the birthplace of Janis Joplin.

But right in the middle of town stands a strategic outpost for Saudi Arabia’s global ambitions, although one that the Saudis appear loath to publicize.

The giant Motiva oil refinery, which just completed a $10 billion expansion that makes it the largest processor of gasoline, diesel and other petroleum products in the United States, is owned by Saudi Aramco and Royal Dutch Shell in a 50-50 partnership.

Saudi Aramco’s investment in the refinery expansion is meant to ensure that Saudi Arabia will retain an important market for its crude in the United States at a time when American politicians are declaring their intention to wean the country off imported oil. Adding to the urgency for the Saudis is the fact that the United States is vastly increasing its production and replacing OPEC crude with that from oil sands in Canada.