Lower prices could eventually crimp some production from higher-cost producers, including recent finds that have spurred a U.S. oil shale boom. Some analysts say output could slow if crude stays below $80 a barrel.
Read MoreCramer: Storm of oil coming our way
In the meantime, countries that have been spending cash generated by $100-a-barrel oil face the much tighter budgets.
An IMF report last month said the hardest hit will include small oil exporters like Oman, which faces a budget gap of 1.8 percent of gross domestic product in 2016. Bahrain could see its budget deficit swell top 5.7 percent by next year, the IMF said.
Bigger producers are also at risk. Oil giant Saudi Arabia faces a budget shortfall of 1.3 percent of GDP in 2017 and currency controls that have led to shortages of many consumer goods, its first since crude exports dried up in 2009 following the global financial crisis.
With oil and gas production accounting for some 70 percent of Russia's government spending, Moscow also faces a big shortfall—after budgeting based on $100-a-barrel oil for 2015. Russia's economic growth was already slowing before the plunge in oil prices. Trade sanctions imposed by the U.S. and Europe—in response to the invasion of the Ukraine—will further crimp growth and government spending.
The impact of budget gaps among big producers like Saudi Arabia and Russia, though, will be softened somewhat by large reserves built up during boom years. But a protracted era of cheap oil would force them to undertake serious belt-tightening.