Almost every polyethylene factory in the United States is on the Gulf Coast. But more than 70 percent of the American plastics manufacturing sector is within 700 miles of Shell’s plant, according to the findings of a 2017 IHS Markit study.

Proximity to markets, lower transportation costs and lower prices for ethane are competitive advantages that strengthened Shell’s decision to proceed, Ms. Mercer said. Shell was also encouraged by a $1.65 billion, 25-year tax reduction package offered by state lawmakers.

The Ohio River plant will subject ethane to high heat and pressure to “crack” the chain of carbon molecules to produce ethylene. When completed and operational in the early 2020s, the Shell plant will turn 1.6 billion gallons of ethane into 3.3 billion pounds of little white polyethylene beads annually.

More than 6,000 tradespeople and laborers will be on the site during the peak summer construction period. Some 600 full-time workers will manage automated technology to operate the completed plant. A 97-mile pipeline from gas separation installations in Ohio and West Virginia will supply ethane; a 250-megawatt gas-fired electrical generating station will power the plant.

The Shell plant is already drawing attention from competitors. In December, Ohio issued air-emissions and water-discharge permits to PTT Global Chemical of Thailand and its partner, South Korea’s Daelim Industrial, for a proposed polyethylene plant in Shadyside, about 80 miles downriver. That plant would be about the same size as Shell’s. Ohio lawmakers are discussing tax incentives valued at more than $1 billion. PTT Global’s decision is expected this year.

China Energy Investment Corporation, the country’s largest energy company, signed a memorandum of understanding with West Virginia in 2017 to invest $83.7 billion in gas-related power, chemical and storage projects in the upper Ohio River Valley. The agreement was the largest among a number of deals that were announced during a summit meeting in Beijing between President Trump and President Xi Jinping of China.

All the activity has generated resistance from environmental and public health groups, which have expressed concern about the effects of the chemical corridor on air and water quality. Emissions of volatile organic chemicals into the air and discharges into the river will increase in an area that already has some of the nation’s worst pollution.