LONDON - Royal Dutch Shell is close to disposing of its stakes in four oil fields in Nigeria as part of plans to cut costs through a $15 billion global assets sale.

On sale is a 30 percent stake in four oil blocks in the Niger Delta -- Oil Mining Licence (OML) 18, 24, 25, 29 -- as well as a key 60 mile long pipeline, the Nembe Creek Trunk Line.

"We have signed sales and purchase agreements for some of the Oil Mining Leases, but not all that we are seeking to divest," Shell said in a statement Wednesday.

The global energy major did not give any details on the value of the deals signed, nor when it expects to complete the sale process.

Quoting two people familiar with the negotiations on with a cluster of prominent Nigerian oil companies, which have emerged as prominent buyers, the Financial Times Wednesday reported that the deal could amount to a value of $5.2 billion.

Buyers for the four blocks have been selected, but two bidders are still negotiating their contracts, FT reported

Shell's consortium partners in the block France's Total and Italy's Eni are also in the process of monetizing their 10 percent and 5 percent stakes respectively in the assets. The remaining 55 percent stake in the four oil fields is held by state-owned Nigerian National Petroleum Corporation (NNPC).

In March, Reuters news agency had reported that Nigerian firms Taleveras and Aiteo had made the highest bid of $2.85 billion for OML 29, the biggest of the four oil fields.

Shell is among an increasing number of global energy companies including Total, Eni, Chevron and ConocoPhillips that have commenced the process of pulling out of oil rich Nigeria, which is plagued with multiple problems, particularly oil thefts.

Another $10 billion worth of assets from international companies are expected to be disposed off in the next few years.

"In June 2013 we announced a strategic review of our operations in the Eastern Niger Delta, which we said could result in the divestment of some of our interests there," Shell Petroleum said.

"A process of staff engagement for those potentially impacted in the event of a successful divestment has begun and will continue over the coming months."

Of the about $15 billion worth of asset sales proposed through 2015, Shell Chief Executive Officer Ben Van Beurden has already completed about $8 billion in sales this year.

The Hague-based company earlier this month sold two natural gas assets in Wyoming and Louisiana for $2.1 billion plus shale acreage in Pennsylvania.