There are five companies on the list that are particularly intriguing: American Electric Power (an electric utility), Exxon Mobil (integrated oil), Gap (apparel), Kimberly-Clark (consumer products) and NextEra Energy (renewable energy).

FORBES: Magazine Article

That gap cannot be closed quickly, since the oil-production business has long lead times and any new oil will take a couple of years at least to come on stream.

ECONOMIST: Pumping all they can? | The

Under these circumstances, diversification among such sources is but a stop-gap solution that can, at best, have a temporary effect on oil supply and, hence, on national security.

CENTERFORSECURITYPOLICY: The Open Fuel Standard

It is a priority because the EU has a huge bill for oil and gas imports from outside Europe - bigger even than the EU's manufacturing trade gap with China, he says.

BBC: EU auditors criticise energy efficiency projects

If oil and gas tax has to fill the gap between other tax and expenditure, it requires some large-scale contingency planning in case it doesn't deliver on forecast.

BBC: Balance of payments