Suddenly, US producers of oil and gas have to worry about market development. Their industry can produce more natural gas than the market needs. They need the market to expand.This is unfamiliar territory. Of course, producers always have had to sell gas. And their industry groups traditionally promote gas use. But the pressure to open new markets for gas has become extreme. The excess of potential deliverability over consumption is large and seems destined to linger. The condition is evident in gas prices that are very low relative to oil despite rising demand. It gives producers good reason to talk about displacing coal in power generation or oil in transportation. But they should tread...