THIS year’s Christmas parade in Lindsay, in the heart of Oklahoma’s oil country, featured the Stars and Stripes every ten yards, 11 horses with riders in Santa hats and a rifle salute by veterans. But the highlight was a thundering, bright red oil tanker covered in fairy lights and owned by Hamm & Phillips, an oil-services firm with local roots that has ridden the shale boom in the state and across America.

That energy revolution is the envy of the business world. Abundant oil and gas have been extracted from underground rocks by blasting them with a mixture of water, chemicals and sand—“fracking”, in the jargon. As well as festive spirit, the firms responsible embody an all-American formula of maverick engineers, bold entrepreneurs and risk-hungry capital markets that no country can match.

Yet now that oil prices have fallen by almost 40% in six months, these firms’ mettle is being tested. Across America shale-shocked executives will spend Christmas overhauling their strategies to cope with life at $70 per barrel, even as investors dump their firms’ shares and bonds. Executives at Lukoil, a big Russian firm, now sniff that shale is like the dotcom bubble—a mania that is being cruelly exposed.

Oil-price slumps usually lead to cuts in energy firms’ investments. Production eventually falls, helping prices to stabilise. In 1999, after the Asian crisis, global investment in oil and gas production dropped by 20%. A decade later, after the financial crisis, investment fell by 10%, then recovered.

This time some of the pain will be taken by the big integrated energy firms, such as Exxon Mobil and Shell. After a decade of throwing shareholders’ cash at prospects in the Arctic and deep tropical waters to little effect, they began cutting budgets in 2013. Long-term projects equivalent to about 3% of global output have been deferred or cancelled, says Oswald Clint of Sanford C. Bernstein, a research firm. Most “majors” assume an oil price of $80 when making plans, so deeper cuts are likely. But much of the burden of adjustment will fall on America’s shale industry. It has been a big swing factor in supply, with output rising from 0.5% of the global total in 2008 to 3.7% today. That has required hefty spending: shale accounted for at least 20% of global investment in oil production last year. Saudi Arabia, the leading member of OPEC, has made clear it will tolerate lower prices in order to do to shale firms’ finances what fracking does to rocks. Even the gods of shale disagree about the industry’s resilience. The boss of Continental Resources, Harold Hamm (whose fortune has dropped by $11 billion since July), has said he can cope as long as the oil price is above $50. Stephen Chazen, who runs Occidental Petroleum, has said the industry is “not healthy” below $70. The uncertainty reflects the diversity of activity. Wells produce different mixes of oil and gas (which sells for less). Transport costs vary: it is cheap to pipe oil from the Eagle Ford play, in Texas, but expensive to shift it by train out of the Bakken formation, in North Dakota. Firms use different engineering techniques to pare costs. Two generalisations can still be made. First, in the very near term, the industry’s economics are good at almost any price. Wells that are producing oil or gas are extraordinarily profitable, because most of the costs are sunk. Taking a sample of eight big independent firms, average operating costs in 2013 were $10-20 per barrel of oil (or equivalent unit of gas) produced—so no shale firm will curtail current production. But the output of shale wells declines rapidly, by 60-70% in their first year, so within a couple of years this oil will stop flowing.

Second, it is far less clear if, at $70 a barrel, the industry can profitably invest in new wells to maintain or boost production. Wood Mackenzie, a research consultancy, estimates that the “break-even price” of American projects is clustered around $65-70, suggesting many are vulnerable (these calculations exclude some sunk costs, such as building roads). If the oil price stays at $70, it estimates investment will be cut by 20% and production growth for America could slow to 10% a year. At $60, investment could drop by as much as half and production growth grind to a halt.