Shell said it was business as usual in the North Sea after announcing plans to sell a collection of ageing assets in the north of the mature province, which looked like routine horse-trading.

However, news that plans to build a new headquarters building at Tullos in Aberdeen have been shelved tacked onto the relevant press release suggested something more significant was going on.

Just nine months ago Kieron McFadyen, then head of Shell's North Sea exploration and production business, pointed to the £25m Tullos project as an earnest example of the giant's commitment.

"We are absolutely committed to the North Sea. I do hope that the statement on Tullos, at least for the time being, puts any doubts to rest," he told The Herald.

While Shell had made no secret of its readiness to sell non-operated assets, controlled by firms with bigger holdings, there was no talk then of selling operated developments like Cormorant Alpha, which went on sale yesterday.

The decision to offload such assets will result in Shell needing fewer people and consequently less office space, and will come as a shock to the workers and builders affected.

They may be the victims of nothing more sinister than the fact that the spreadsheets used to decide which assets to invest in produced a different result this year than in 2006.

Changes in a numbers of significant variables, including the cost of infrastructure, services, workers and the price of oil, all could have an impact.

Having spent years warning that increases in North Sea taxation in recent years would spark a flight from the province, Oil and Gas UK yesterday said Shell's move highlighted the need for fiscal stability.

The industry lobby group is also concerned that the potentially huge costs involved in decommissioning older assets could have a big impact on thinking at firms such as Shell.

These are factors that have affected other giants which have been refining their North Sea portfolios for years, and shifting resources into the development of mega-projects in emerging areas like south-west Africa.

Exxon Mobil, the American colossus, part-owns some of the assets put on the block by Shell yesterday.

However, none have done anything to compare with Shell's move in the drama stakes this year. Bitter rival BP is in the course of building a headquarters in the Dyce heartland of the Aberdeen oil-and-gas patch.

New chief executive Tony Hayward may not be in a hurry to make big changes in the North Sea while BP grapples with the challenges of operating in Russia, which has been the main driver of production growth in recent years.

Ironically, Shell may feel more prepared to sell off worthy but dull North Sea interests after agreeing a deal to "sell" half its holding in the key Sakhalin-2 development off Siberia to Gazprom, the Russian giant, recently.

Chief executive Jeroen van der Veer has made it clear that he believes the company should focus increasingly on such blockbuster assets, in pursuit of its strategy of "more upstream, profitable downstream".

But this only means that one super-major may be singing the familiar "big is beautiful" theme more lustily than others that may not have accumulated as many North Sea assets as Shell in the past.

Even if yesterday's move signals the beginning of the end for Shell in the North Sea it would be a mistake to assume that that means the end is nigh for the province.

Many well-paid specialists are now working for the fleet-footed independents like Venture Production that have been formed in recent years with the express purpose of taking over assets from the likes of Shell.

These have found they can make plenty of money out of fields that do not meet the return targets set by giants.

Mike Wagstaff, chief executive of Venture Production, recently noted that, tax rises notwithstanding, the region offers a stability that firms operating in places like Russia can only dream of.

Many western firms are desperate to build positions in the Middle East but are stymied by the fact that with demand for energy soaring the sultanates feel they can manage fine without outside help.

The resultant strong prices mean that firms are working flat out to pump as much as they can out of the North Sea, creating a boom in demand for services firms.

Firms like Wood Group that can help maximise production from the kind of relatively small fields that Shell is selling are in especially big demand.

More significantly, demand for North Sea acreage has been booming, with exploration licensing rounds attracting record levels of interest.

Based on current levels of production, there may only be seven years worth of oil and gas reserves in the North Sea, assuming no more reserves are found and exploration and production technology stands still.

However, with around 27 billion barrels oil equivalent still under the North Sea on official estimates, there could be about 40 years worth of hydrocarbons left. That leaves lots to play for, and not just by industry dreamers.

The fact that both Venture and Faroe Petroleum recently won hefty funding pledges for North Sea investment from US financiers made it clear that some very hard-nosed analysts think it is not yet time to call time on the province.