India's top oil and gas producer, Oil and Natural Gas Corp., plans to spend 100 billion rupees over next five years to help maintain output from its ageing fields, a top official said.

ONGC, India's second-most valuable company with a market capitalisation of $48 billion, is facing a decline in output from its onshore fields that were discovered decades ago.

"All the fields which we had discovered in the '60s and '70s are quite matured and old," Ajit Kumar Hazarika, director for ONGC's onshore assets, told Reuters in an interview on Tuesday.

"We have not discovered any new field in the last 20-22 years," he said.

Hazarika, who runs ONGC's seven onshore assets, aims to produce 43 million tonnes of oil and nearly 29 billion cubic metres of gas over next five years but it will not be easy.

"This is a big challenge, unless we find a new field it is very difficult to increase the production from old fields because the natural decline itself is 7 to 8 percent," he said.

The state-run firm had produced 42 million tonnes of oil from its onshore fields in the past five years.

Hazarika said ONGC was trying to adopt new technology such as reservoir management and has appointed consultants to help arrest the decline in its onshore fields.

"Our efforts are on and whatever we do now, the fruits may come after one or two years," he said.

ONGC produces the bulk of its oil and gas from its offshore fields.

In the fiscal year ended March 2007, ONGC's onshore oil output accounted for one-third of its total production of 26.05 million tonnes, while gas output stood at about 6 billion cubic metres of the total 22.4 billion cubic metres.