Bob Dunston, 60, who was an engineer supervisor, had worked for 18 years at Weatherford International, one of the world’s largest oil field services companies. On April 9, he found himself on a Skype call with his boss’s boss and a representative from human resources.

“He said, ‘Hey, you’ve been a great employee. You’re one of my best employees, and this and that, but with oil the way it is right now, at 20 bucks a barrel, we’re just having to reduce the division head count,’” said Mr. Dunston, who had been working from home during the local coronavirus lockdown.

Mr. Dunston is still trying to recover from Hurricane Harvey in 2017, which pushed 15 feet of floodwaters into his two-story townhome. In a new home, but without a job, he is unsure what is next.

“My game plan may be just retire,” Mr. Dunston said. “I saved money for a reason.”

Houston is no longer the oil town it was in past decades. Its economy has diversified, as has its population. More so than cowboy boots, scrubs are its unofficial uniform; the Texas Medical Center is the largest medical complex in the world, with more than 100,000 health care workers.

But much of Houston’s energy still comes from energy.

Business owners of all types make it a daily habit to check the price of West Texas intermediate, the benchmark for U.S. crude oil prices. The Houston Marathon is officially known as the Chevron Houston Marathon. One of the city’s most respected disaster-recovery leaders, whom the mayor recently named as his Covid-19 economic recovery czar, is Marvin E. Odum, the former president of Shell Oil.

Houston’s cultural institutions and nonprofit groups rely on philanthropy from the oil and gas industry. The city has had world-class opera, theater, ballet, symphony and art in large part because it had world-class oil money.

Now, all of that is under threat.

“The oil and gas industry employs about a quarter of a million people in Houston, so we’re looking at losing at least 10 percent of those jobs, if not more,” said Patrick Jankowski, senior vice president of research at the Greater Houston Partnership, a regional business association. “Right now, I think people are probably more worried about their job than they are about Covid-19.”