Oil and natural gas companies scrambled on Monday to determine the extent of damage to refineries, pipelines and offshore platforms in the immediate aftermath of the former Hurricane Ida, which made landfall in Louisiana packing winds near 150 mph.

Ida battered Louisiana Sunday night as a Category 4 hurricane, delivering storm surges and flooding rains. It left more than one million homes in the state without electricity, according to Poweroutage.us., which aggregates live data from utilities. The storm pummeled New Orleans on the anniversary of Hurricane Katrina, which devastated the city 16 years earlier.

Entergy Corp., which serves New Orleans, cited “catastrophic wind damage” and said some neighborhoods of the city could go without power for weeks.

At the same time,...