The shale oil boom has transformed the U.S. and global energy sector to such an extent that it has upended traditional supply dynamics and made forecasts far more polarized. Investment banks, many of which finance new projects, along with oil majors such as Total and Eni, have warned that huge spending cuts caused by a plunge in oil prices since 2014 would lead to a supply crunch in the next two years. Yet Goldman Sachs, the only bank to make more than $1 billion a year from commodities trading, believes a looming recovery in U.S. output on the back of...