Oil and Natural Gas Corporation, India's largest oil producer, has made a huge gas discovery in deep-sea Bay of Bengal, with initial reserves being assessed to be in the range of 4 to 6 trillion cubic feet.

ONGC struck gas in the Krishna Godavari Basin block KG-DWN-98/2, which lies adjacent to Reliance Industries' D6 block, where 11.9 trillion cubic feet of certified in place gas reserves have been struck till date.

Officials said the gas find could be at least half the size of Reliance discovery. "It's a good find. We have struck 60 meters thick gas pay zone (reserve strip) in one well."

While Petroleum Minister Mani Shankar Aiyar, who is in Oslo, confirmed the gas find, ONGC director (offshore) N K Mitra said: "A lot of formalities have to be completed for an announcement about the potential reserves can be made."

"We are governed by the rules of the Securities and Exchange Board of India and the reserves have to be first tested and then verified by Directorate General of Hydrocarbons (India's upstream regulator) before they can be quantified and announcements made," he said.

ONGC had acquired 90 per cent stake in the Block KG-DWN-98/2 from Cairn Energy of the United Kingdom for $135 million in 2004. Cairn Energy, which still holds 10 per cent stake, had in 2001 made oil and gas finds at four wells in the block -- gas at prospect 'N', gas at prospect 'R' (now named Annapura), oil and gas in prospect 'P' and oil at prospect 'M'.

Cairn had established 0.75 to 1 trillion cubic feet of gas reserves in Annapura and at least 200 million barrels of oil reserves in prospects 'P' and 'M'.

The previous finds and the latest find together would make up for at least 6 trillion cubic feet of gas reserves, the official said.