HOUSTON—The frail and elderly were put aboard buses Wednesday and authorities warned 1 million others to flee inland as Hurricane Ike steamed toward a swath of the Texas coast that includes the nation’s largest concentration of refineries and chemical plants.

Drawing energy from the warm waters of the Gulf of Mexico, the strengthening storm was expected to blow ashore early Saturday somewhere between Corpus Christi and Houston, with some forecasts saying it could become a fearsome Category 4, with winds of at least 131 mph.

Such a storm could cause a storm surge of 18 feet in Matagorda Bay and 4 to 8 feet in Galveston Bay, emergency officials warned. The surge in Galveston Bay could push floodwaters into Houston, damaging areas that include the nation’s biggest refinery and NASA’s Johnson Space Center.

Four counties south and east of Houston announced mandatory or voluntary evacuations, and authorities began moving weak and chronically ill patients by bus to San Antonio, about 190 miles from Houston. No immediate evacuations were ordered in Harris County, which includes Houston.

Johnny Greer, a 54-year-old retired plant operator at Dow Chemical Corp., boarded up his house a mile from the Gulf of Mexico in Brazoria County and planned to hit the road.

“Gas and stuff is high. But you can’t look at all that,” he said. “I think my life is more valuable than high gas prices.”

About 1 million people live in the coastal counties between Corpus Christi and Galveston. An additional 4 million live in the Houston area, to the north.

The oil and gas industry watched the storm closely, fearing damage to the very heart of its operations.

Texas is home to 26 refineries that account for one-fourth of U.S. refining capacity, and most are clustered along the Gulf Coast in such places as Houston, Port Arthur and Corpus Christi. Exxon Mobil Corp.’s plant in Baytown, outside Houston, is the nation’s largest refinery. Dow Chemical has a huge operation just north of Corpus Christi.

Refineries are built to withstand high winds, but flooding can disrupt operations and—as happened in Louisiana after Hurricane Gustav—power outages can shut down equipment for days or weeks. An extended shutdown could lead to higher gasoline prices.

As always, some hardened old-timers decided to ride it out. Fourth-generation fisherman James Driggers, 47, planned to spend the storm aboard his 80-foot boat docked in Freeport.