The world's largest oil exporter, Saudi Aramco, will increase light sour crude output from two fields - Shaybah and Khurais - by 550,000 barrels per day (bpd) in 2016-2017, to rebalance its crude quality and extend the lifespan of mature fields.

The expansions of the fields will start up "within a few months of each other," Chief Executive Khalid al-Falih said on Sunday.

Shaybah would come first, by the end of 2016 or early 2017, the head of Saudi Arabia's national oil company told reporters on the sidelines of the World Energy Congress, confirming a July report from Thomson Reuters on the expansion.

"This will bring it up to a million barrels (per day). We're in the process of awarding the contract in the next few days," he said, adding that an ongoing project at Shaybah will also add 250,000 bpd of natural gas liquids output in end-2014.

The Khurais expansion project was at the front-end engineering stage and the expansion to increase the field's output by 300,000 bpd to 1.5 million bpd should be completed by 2017, he said.

"This will basically allow us to relax production from the more matured fields and reservoirs and extend them and also to rebalance our crude slate," al-Falih said.

Referring to the giant new Manifa oilfield, which started production earlier this year, he said "we're tilting towards more heavy (crude)."

With the expansion, Saudi Aramco's exports will be predominantly light crude as heavy sour oil from Manifa will be processed at domestic refineries, al-Falih said.

Saudi Aramco will turn into a major refined products supplier to Europe and Asia in the next five years as it completes another two 400,000 bpd refineries, including the latest project in Jizan, which is likely to be delayed by 6-12 months because work on associated infrastructure is behind schedule.

"We hope we can keep it on track," al-Falih said, adding that the project is scheduled to start up in late 2016 to early 2017.