Aberdeen University petroleum economist Professor Alex Kemp said the two deals were a “big step” – and more could follow from China.
“When they make an investment like that they will look around for more opportunities,” he said.
“It is a big step – let’s hope they do some exploration.”
However, Andy Brogan, global transaction advisory services leader for oil and gas at Ernst and Young, said: “There may be more activity from the foreign national oil companies in the North Sea, but their focus is likely to be elsewhere, particularly in Africa and South America.
“Where they do invest, they tend to want to grow production, so their arrival should be positive.
“It’s interesting to note that in both cases the buyers are trying to keep incumbent management in place. This shows a recognition of the importance of North Sea know-how.”
Eddie Leigh, managing director at Simmons and Company International in Aberdeen, said: “This is a strategically-significant deal for CNOOC as it will secure reserves and production from high-quality assets in key international producing regions and also brings with it significant management capability.
“This announcement, together with Sinopec’s tie-up with Talisman UK, demonstrates that the Chinese are working hard to secure their oil and gas supply lines. These deals should see considerable new investment in the assets concerned.”
Malcolm Webb, chief executive of industry body Oil and Gas UK, said: “We would expect investment activity to remain high. A diverse range of investors will help maximise recovery of the UK’s substantial remaining oil and gas resource to the benefit of jobs, tax revenues, export earnings and energy security.”
Politicians from across the party divide welcomed news that two Chinese firms have invested in the North Sea.
The SNP, Labour and Conservatives said the news was a sign of confidence in Scotland’s energy sector.
Energy Minister Fergus Ewing said: “With more than half of the value of the North Sea’s oil and gas reserves yet to be extracted, 24billion barrels with a wholesale value of £1.5trillion, oil and gas will remain an enormous economic resource for decades.
“Rising capital investment reaching £8.5billion in 2011, and expected to rise to £11.5billion in 2012, demonstrates the confidence investors and the industry have in Scotland.”
UK Energy Minister Charles Hendry said: “It reinforces the attractiveness of this sector.”
Shadow energy minister Labour MP Tom Greatrex said he was pleased the firms expected no job losses.
Conservative MSP Murdo Fraser said: “This underlines the strength of the Scottish oil and gas industry.”