GLOBAL oil markets have historically been prone to epic cycles of boom and bust. Because it generally takes years for a fresh exploration project to yield its first barrels, producers tend to over-invest when prices are high, only to see the value of their output crash once a large number of new fields begin operation. That forces companies to cut back on their capital expenditure, reducing growth in supply until prices rise again and the cycle begins anew.

To an extent, this familiar pattern seems to have repeated itself in recent years. Benchmark oil prices fell by more than 60% in 2014, and have oscillated at relatively low levels since then. During the past year, however, they have climbed by roughly half, and are now approaching the $70 level, which has not been breached since 2014.

However, the chances of another meteoric run-up like that of 2008, when the price reached $147 a barrel, are low. According to calculations by Rystad Energy, a consultancy, the cheapest type of new oil field to develop is offshore exploration on continental shelves. Its break-even price—the amount of revenue per barrel at which a project starts to be profitable—ranges from just under $20 to a bit below $60. Development costs are slightly higher in deeper waters. In the most difficult environments, like extracting and refining the heavy oil found in the the tar sands of Canada’s frozen north, the break-even threshold is nearly $90 a barrel. Any new exploration in these regions would take years to come online.

Shale oil, however, runs on a different time-frame. Even the most costly shale projects break even at around $70 a barrel, and new wells can be drilled and start producing within a matter of months. As global oil prices have recovered, America’s shale output has rebounded sharply: this year, the country’s total production reached almost 10m barrels per day, a level last attained in 1970. Companies in the United States can profitably generate vast quantities of fresh oil supply at the current world price. If the market continues on its current trajectory, America could realistically become a net exporter of oil for the first time since 1953.