Saudi Aramco will invest $15 billion for buying 20 per cent in the Reliance Industries oil-to-chemicals division.

Saudi Aramco will also supply 5,00,000 barrels of crude oil a day on a long-term basis to Reliance Industries' Jamnagar refinery, Mr Ambani said.

"This signifies perfect synergy between the world's largest oil producer and the world's largest integrated refinery and petrochemicals complex," he added.

The deal should be completed by March and is subject to due diligence, definitive agreements and regulatory and other approvals, the billionaire added.

Aramco has been targeting refining deals in India since at least last year, when chief executive officer Amin Nasser told reporters that the firm wanted to double its capacity to produce gasoline and other fuels.

Saudi Aramco, the world's biggest oil producer, has been in talks with Reliance Industries on the stake sale as it plans to secure new markets for its crude.

This partnership will cover all of Reliance Industries' refining and petrochemicals assets, including 51 per cent of the Petroleum Retail JV.

"Since its inception, our Jamnagar refinery has been processing Saudi oil every single day for 20 years. Now we have transformed our longstanding relationship of two decades, based on mutual trust, into a Partnership of growth potential for many more years. I feel privileged to welcome Saudi Aramco, which is the largest business enterprise in the world," Mr Ambani said.

"I am grateful to the visionary leaders, Honourable Prime Minister Shri Narendra Modi and His Royal Highness Crown Prince Mohammad bin Salman Al Saud, who have laid the foundation for strategic cooperation between India and the Kingdom of Saudi Arabia," he added.