Ineos has acquired the oil and gas business from Denmark's Dong Energy for over $1bn (£771m).
Jim Ratcliffe's chemical firm will pay an initial $1.05bn for the business, with up to $250m in further payments relating to a stabilisation plant and a new North Sea oil field development.
Around 440 employees at Dong Energy will join Ineos when the sale goes through, which is expected to be within the third quarter of this year. Ineos said the new business has a strong portfolio of long-life assets, producing 100,000 barrels of oil equivalent per day last year.
It added that the deal will also complement Ineos' existing upstream activities, formed on the acquisition of Dea's and part of Fairfield's UK portfolio in 2015.
Jim Ratcliffe, Ineos chairman said: "Dong Energy's oil and gas business is a natural fit for Ineos as we continue to expand our upstream interests. This business is very important to us at this stage of our growth plans and we are delighted with the expertise that comes with it.
"We have been successful in our petro-chemical businesses, focussing on operating our assets safely, efficiently and reliably and we intend to do the same with our oil and gas assets."
We are keen on further growth and already see lots of opportunity within this impressive portfolio when it transfers to Ineos.
The deal comes as Dong Energy focuses its efforts on offshore wind power. It announced plans to withdraw from its oil and gas business back in November, to concentrate on renewable energy.
The former state-controlled utility, which listed on the Copenhagen Stock Exchange in June last year, said it had been looking for the best transaction to ensure "the optimal conditions for the long-term development of the oil and gas business".