* Aramco could reduce stake in Cilacap facility -Indonesia govt

* Aramco, Pertamina decline to comment

* Indonesia hopes to formalise joint venture in Oct

(Adds comment, background)

By Wilda Asmarini

JAKARTA, Aug 24 (Reuters) - Oil and gas giant Saudi Aramco may reduce its stake in a proposed $5.5 billion refinery project in Indonesia, the southeast Asian nation's energy minister said on Wednesday.

Aramco has asked to cut its share of the project to upgrade a refinery in Cilacap in the province of Central Java to 30 percent from 45 percent, interim energy minister Luhut Pandjaitan told reporters.

He added that state energy company Pertamina , which is jointly developing the facility, would likely absorb the difference.

"Aramco's progress has been rather slow. We want to chase this, not just talk. We want something concrete now," he said.

Saudi Aramco declined to comment on Wednesday, while a spokeswoman for Pertamina in Jakarta said she could not make immediate comment.

Indonesia hopes to formalise a joint venture and other details of the partnership between Aramco and Pertamina during a visit from Saudi King Salman bin Abdulaziz to the archipelago in October.

Forecasting growing crude oil demand, Saudi Aramco has been looking to invest further in Indonesia's refining and petrochemicals sector, part of broader expansion plans in China, India, Vietnam and the United States.

The Cilacap upgrade is expected to increase the refinery's crude processing capacity to 370,000 barrels per day (bpd) from 348,000 bpd at present, and is targetted for completion by the end of 2022.

(Writing by Fergus Jensen; Editing by Joseph Radford) ((fergus.jensen@thomsonreuters.com; +62 21 2992 7604; Reuters Messaging: fergus.jensen.thomsonreuters.com@reuters.net))