A senior Opec official on Monday highlighted the likelihood that a reduction in the oil cartel’s production would be filled by an increase in US shale oil output. The comments by Abdalla El-Badri, secretary-general of Opec, were echoed by the head the International Energy Agency, underlining the limits to any rally in the oil market. Production from US shale was a primary force behind the global oil glut that has sent oil prices down about 70 percent since the summer of 2014. Opec, led by its most influential member Saudi Arabia, in the second half of that year, abandoned its role as a throttle on supply in an attempt to avoid losing market share to US shale, precipitating the price collapse. Yet even though shale drilling has slumped, US oil output has been surprisingly resilient. “I don’t know how we are going to live together,” Mr El-Badri said of the US shale industry. “Any increase in the shale oil will come immediately and cover any reduction.” Mr El-Badri was speaking at IHS CERAWeek, an energy industry conference in Houston. Fatih Birol, executive director of the IEA, said scale would hold back price gains even as drilling slows.