"We are working on a development plan and investment approvals will be sought in a month's time," ONGC Director (Offshore) Tapas Kumar Sengupta told PTI here.
Production from the fields is targeted to start in 2019 with an output of 10,000 barrels per day initially, he said.
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The Cabinet Committee on Economic Affairs (CCEA), headed by Prime Minister Narendra Modi, had in March decided to return the fields to its original licensee after the contract with Essar could not be concluded even after 20 years.
Essar Oil and Premier Oil of UK were awarded the Ratna and R-Series oil and gas fields, lying 130 km off the Mumbai coast, in December 1996 but the production sharing contract (PSC) could not be signed due to differences in rates of royalty and other issues.
"We have taken over the fields and have started the process of developing them," he said.
The Ratna and R-series oil fields hold an estimated 87 million barrels of oil and 1.2 billion cubic metres of gas reserves.
The medium-sized Ratna and R-Series (R&RS) fields have been languishing since 1993 when the then P V Narasimha Rao-led Congress government decided to invite bids.
The fields were awarded to a consortium led by Essar Oil in 1996. The Cabinet Committee on Economic Affairs (CCEA) in 1999 approved finalising and concluding PSC within six months after negotiations were held by the negotiating team of secretaries (NTS), but the same has not been signed till date over royalty and cess amount to be charged from Essar Oil and partners.
NTS held 20 meetings between November 1999 and June 2013 and kept setting targets for completion of negotiations and signing of production sharing contract, but it could not stick to its own targets for completion of the negotiations.
ONGC had discovered the fields and created facilities in Ratna R-12, which is part of R&RS, at a cost of Rs 472.55 crore. These facilities were used by the company for production since 1983 before production was stopped in September 1994 after the field was put up for auction, he said.
These facilities have been deteriorating as a result of 'plundering and looting' of platform utilities and equipment. The estimated repair cost for the existing facilities would be Rs 1,085.70 crore.
"We have to build the infrastructure afresh," Sengupta said.
The fields have been reverted to ONGC subject to payment of cess and royalty at the current rates.