Ann: Asset Value Review and Other Items, page-14

The impairment is just an accounting entry, it doesn't really mean anything.

The original asset values would have been derived from extrapolating oil prices at the time out into perpetuity.

The new values (post impairment) will now be extrapolating the current oil price out into the future (assuming it will be at this current price forever).

The reason this occurs is because no Auditor would ever be willing to make the judgment: "no no, I am happy to predict oil prices will normalise & increase in a few years - and I'll put this in writing, to be used as assurance to a broad range of stakeholders". Instead, they protect themselves and base the valuations on the prevailing observable market prices. Can't ever be in the wrong for taking the current observable price. Would be ludicrous to do this any other way, given the potential liability on auditors.

I personally do not think oil prices right now reflects the price forever (hence my holdings in WPL), I think there's been a big external shock that's thrown the price out of whack for the short/medium term (though I could be wrong, this is just my thesis). If this is the case, the true value of the assets remain largely intact, despite whatever journal entry may or may not be plugged into their accounts.