The deal will give Serica its first production in the UK, where the company says it wants to build a stronger presence.
The acquisition is the latest in a series involving oil and gas firms and investors based in London buying assets in the North Sea.
Some believe the gradual retreat of majors like BP from mature areas of the territory is creating opportunities to acquire assets at good prices.
BP has been selling off non-core North Sea assets to focus investment on big new fields west of Shetland.
It is investing in these alongside Royal Dutch Shell, which has put some North Sea assets up for sale.
Shell yesterday announced plans to sell the bulk of its stake in Australia's Woodside Petroleum for around $5bn (£2.9bn) after tax.
This forms part of new chief executive Ben van Beurden's plans to sell $15bn of assets.
Tony Craven Walker, chairman of Serica Energy, said: "The agreement to buy an interest in the producing Erskine field from BP is an important step for Serica."
The deal will give Serica its first production from the North Sea and provide what Mr Walker described as a valuable income stream for Serica. He noted the tax advantages the deal would generate for Serica, which has North Sea acreage containing the undeveloped Columbus discovery and exploration prospects.
The company will be able to set the losses it has incurred on its North Sea activity against the revenues generated from its stake in Erskine.
Mr Walker said the deal could boost the value of its existing North Sea position by increasing the chances the Columbus find could be brought onstream.
Erskine produces via the Lomond facilities which lie close to Columbus.
"Securing an interest in Erskine provides us with a valuable foothold in a nearby producing field together with access to associated infrastructure and will assist us in our ongoing efforts to bring Columbus on to production," said Mr Walker.
Last week Mark Routh, who runs London-based Independent Oil & Gas, said the company would pursue such a North Sea hub strategy and highlighted its appetite for deals.
In May, London-based Trap Oil linked up with America's Prostar investment business to hunt for North Sea assets. Prostar has raised around £60m to invest in deals.
Aim-listed Serica will pay BP $11.1m (£6.5m) cash and 27 million shares, subject to final adjustment.
BP has agreed to hold the shares for not less than one year. They will represent 9.7% of the enlarged company.
Shares in Serica Energy closed up 0.25p at 11.12p.
Serica said net production for the year 2014 accruing to the interest in Erskine acquired is estimated to be 1,234 barrels oil equivalent per day.
The asset generated $10.2m post tax profit in 2013.
Shell will reduce its stake in Woodside to 4.5% from 23.1%.
Woodside will buy back half the shares Shell is selling. The rest will be sold to institutions.