(Bloomberg) -- Drillers cut exploration in U.S. oil fields to a 15-year low as billions of barrels from old discoveries became worthless and explorers abandoned growth plans.

The number of active oil rigs in the U.S. fell by four to 176, the lowest since 2005, according to Baker Hughes Co. data released Friday. Energy companies have been parking rigs on an almost uninterrupted streak for more than four and half months.

Stung by the pandemic-driven slump in demand and prices, oil explorers are fleeing from the very lifeblood of their business: Drilling for new discoveries. Exxon Mobil Corp. and Chevron Corp. have warned they probably will wipe billions of barrels of reserves from their books because weak prices have made them unprofitable to pump.