A gauge of global commodities prices may drop 20 percent over six months as rising supplies drive oil and copper lower, according to Goldman Sachs Group Inc.

While prices rallied as money flowed into exchange-traded funds tied to oil, markets are well supplied and crude stockpiles are forecast to continue rising, analysts including Jeffrey Currie in New York wrote in a report dated March 13. Commodities investors will see total returns of 4.5 percent if they retain their holdings for 12 months, the bank said.

“The recent oil exchange-traded fund inflows are arriving too early and could reverse,” the analysts wrote. “We don’t believe prices have reached a trough yet.”

The Standard & Poor’s GSCI commodity index has rebounded 8 percent from the lowest in almost six years in January as crude rallied on speculation that demand is improving and a slowdown in U.S. oil output will shrink a global supply glut. Still, U.S. inventories and production are at the highest levels in more than three decades and Oman estimates the market is oversupplied by 1 million to 1.5 million barrels a day.

Goldman forecasts that the S&P GSCI Enhanced Total Returns commodity index, which takes into account gains or losses from rolling futures contracts forward as well as changes in the underlying price, will slide 18 percent over the next three months and 20 percent over 6 months. The index is down 6 percent this year. The Bloomberg Commodity Index of 22 raw materials is 5.3 percent lower in 2015.

U.S. inventories will continue to build at a “rapid pace,” according to Goldman. Stockpiles have risen for nine weeks through March 6 to 448.9 million barrels, the most in Energy Information Administration records dating back to August 1982. The nation pumped 9.37 million barrels a day, the fastest pace in data compiled by the Energy Department’s statistical arm since January 1983.

Drilling Rigs

The number of rigs drilling for oil in the U.S. was at 922 as of March 6, dropping for a 13th week, according to data from Baker Hughes Inc., a services company.

The rig count needs to fall as much as 30 percent further and U.S. benchmark oil prices need to drop to $40 a barrel over the first half of 2015 for the market to rebalance itself, Goldman said. West Texas Intermediate crude for April delivery was at $46.70 a barrel in electronic trading on the New York Mercantile Exchange, down 35 cents, at 10:46 a.m. London time. Brent in London was at $56.60.

Goldman also projects copper prices to extend declines amid weaker demand from China, the world’s top consumer, and as inventories of the metal climb this year and in 2016. China’s industrial output, investment and retail-sales growth missed analysts’ estimates in January and February, signaling the country is behind its economic targets.

Copper for delivery in three months was down 0.4 percent at $5,818.50 a metric ton on the London Metal Exchange. Inventories rose 0.9 percent to 333,575 tons, the highest since January 2014.