The tussle for supremacy between OPEC and U.S. shale drillers is killing off older oil fields at the fastest pace in almost a quarter century. That could hurt the industry once the current glut has faded.  The three-year price slump triggered by the battle for market share choked off funds for aging deposits elsewhere, accelerating their decline. Output at older fields from China to North America — making up a third of world supply — fell 5.7 percent last year, the most since 1992, according to Rystad Energy AS. It’ll drop about 6 percent in 2017 if oil stays at current prices, the consultant said.  Oil fell from above $100 a barrel in 2014 to as low as $26 in 2016 as the Organization of Petroleum Exporting Countries opened the taps in an effort to stem the surge in shale production. That set off the worst industry downturn in a generation, forcing cost-cutting companies to focus on higher-margin assets at the expense of older, costlier fields. While OPEC changed course last year and curbed output to boost prices, shale was the main beneficiary and resurgent U.S. output has kept crude below $50.  “A lot of the focus is on OPEC and shale and not on the decline at these mature fields, where supply is struggling,” said Espen Erlingsen, a partner at Oslo-based Rystad. “We’re starting to see the long-term impact of lower oil prices.” Though new projects mean total global production continues to rise, the slide at aging fields may give OPEC a helping hand by reducing surplus supply today, according to Erlingsen. The danger for major oil companies — many of which are gathering in Istanbul this week for the World Petroleum Congress — is that the decline may be difficult to reverse, increasing the risk of future supply shortfalls as spending cuts take their toll for years to come.  “About $1 trillion in investments has been lost in the current downturn,” Saudi Aramco Chief Executive Officer Amin Nasser said Monday at the congress. “Conservative estimates suggest we need about 20 million additional barrels a day over the next five years.”

Fields in Decline

Oil deposits go through a number of phases, with production initially rising before flattening out and eventually waning as the reservoir pressure drops. About a third of global output comes from mature conventional fields — about 30 million barrels a day, or around three times Saudi Arabia’s supply — according to Erlingsen. Their fast-declining supply “is making OPEC’s life a little easier,” he said.