More than three-quarters of U.S. oil wells, collectively, make just 6% of the country's oil. They're called marginal wells because of how small their output is. But they're a big deal.

OMV (Norge) AS has plugged the Horatio prospect in the North Sea. The well is classified as dry. Well 35/6-6, the first to be drilled in production license 1109, was drilled using the Transocean Norge ...