Laying the investment pipeline
It is not an easy one to explain for representatives of the oil and gas industry, though they are giving it their best shot.
Last March, George Osborne stunned them with an extra £2bn in tax, and, predictably, they howled, with dire warnings of the consequences in reduced investment.
They continue to howl. "Betrayal" is how the French boss of Total E&P put it to a gathering of industry leaders in Edinburgh last week.
But they also continue to invest. And some.
With Brent crude reaching $125 per barrel this week - higher, in sterling terms, than the $147 peak reached in 2008 (remember the depreciation since then?) - it is not hard to see why.
The latest figures from Oil & Gas UK, which speaks for the industry, show investment hit £8.5bn last year and is forecast to be £11.5bn in 2012.
The Treasury said this might happen, in response to the loud complaints about the tax rise after last year's Budget.
It was pointed out that Gordon Brown, when he was Chancellor of the Exchequer, had twice raided oil producers' profits in the past decade, and it did not seem to hurt subsequent investment.
So the industry is now arguing that investment levels are not as much as they need to be, if the UK is to unlock available resources under the seabed.
It claims that investment level is not as much as it would be without the tax.
It says the return on capital has fallen by two thirds in the past decade, as a simple measure of oil flowing per pound invested, so more has to be done simply to stand still.
And it contends that the big sum is distorted by very large lumps of investment, mainly for big fields to the west of Shetland, to which companies were already committed before the Budget shock.
Some 85% of investment last year was in only seven fields.
They are supported with a longer-term view from oil economist Tony Mackay, whose latest forecasts coincide with the industry's annual survey of the previous year's activity.
He is telling an audience in Aberdeen today that investment will indeed remain high, and peak at £8.5bn in 2014, before a steep decline sets in, to reach £4.3bn in 2020.
His take on that is that suppliers have to prepare for the decline by shifting their attention to servicing the fast increase in offshore renewable energy.
Pump it or lose it
Meanwhile, a key part of the hydrocarbon industry's lobbying case to the Treasury goes that the North Sea is now dependent on some rather elderly kit, and if investment does not go into maintenance of it, the government should know that this is not something that can be brought back to operational life with a scoosh of WD40 in a decade or so.
Put simply: incentivise us to use it or we all lose it.
And in fields very substantially smaller than the Klondike days of the 70s and 80s, industry bosses say it is not worth investing with new equipment, however clever the oil engineers become with their tie-backs, horizontal drilling and enhanced field recovery.
Another reason for giving the producers a hand to keep the older equipment pumping.
However, the other side of the repair and maintenance argument that is spelled out from the annual survey of activity from Oil & Gas UK is that keeping it operational means a lot less work involved in decommissioning.
There have been some eye-popping figures for the extent of work to take all those platforms, topsides and pipelines out of the North Sea when they are finished.
They remain big.
The industry body reckons the existing equipment will cost £28.7bn to decommission, and the kit now being added to the inventory will increase that by around £4.3bn.
But the point at which that turns into a huge business opportunity is receding.
Work may continue at a modest pace for the next four years, but the real bonanza for the wrecking crews with the blow-torches is now forecast to start as late as 2025.
The evidence from its annual survey that backs up Oil & Gas UK more convincingly is the number of new drilling wells sunk last year - at 15, down 50% on the previous year, and the lowest number since the North Sea started to give up its black riches more than 40 years ago.
Only 28 appraisal wells were sunk.
The new targets had a high strike rate of 60% success, suggesting explorers were going after the easier targets.
They may also have been constrained by some sharp increases in the cost of hiring drill equipment.
Such is the urgency of finding deposits under other seas that the staggering price of hiring a drilling rig (a semi-submersible rig costs $273,000 per day, and that just buys you "standard configuration") makes the likely returns on UK waters seem relatively unattractive.
And that's the main thrust of the industry's point to George Osborne as he writes his 2012 budget for delivery on 21 March - the industry does not need UK oil, when there are big prizes elsewhere, so if he wants to maximise the 23bn barrels thought to be recoverable from under UK waters, he'll have to do more to persuade them to stay.
A good start with be extending tax breaks on particular categories of field.
The lobby operations cites 3bn barrels of oil it describes as "fiscally stranded" - accounting imagery that summons a beached whale to mind, as if the only human option is for Mr Osborne to give it a tow out to sea.
It is likely the Treasury will be more concerned at the current production rates from the UK Continental Shelf (the public accounting device designed to avoid any impression that the oil might belong to, for the sake of argument, Scotland).
Production slumped by 18% last year, due to unplanned stoppages and a low level of new reserves coming on stream.
That's output lost to the economy, which Oil & Gas UK reckons explains about 0.2% less in gross domestic product.
And if energy demand were to be steady, that's more that needs imported.
Contrast that with Norway. Quite apart from the recurring debate about the ability of Scotland to emulate Norway's oil fund by stashing away financial surpluses, the Norwegians are enjoying some spectacular new finds.
And because the Oslo government owns Statoil, they're getting the benefits of its activities not only in Norwegian waters, but with announcements in recent days of significant fields off Tanzania and Brazil.
The company has commissioned two new drilling rigs to tackle its northern waters.
It may be that Norway's got lucky, with a second wind for the North Sea.
But remember: you often have to work hard to get lucky.