One of Texas’ oldest oil fields, in decline for decades, has become one of the hottest places in the country to drill for crude, as energy companies create clusters of wells with layers of horizontal branches. The Permian Basin—86,000 square miles centered on Midland, Texas—has been pumping oil since the 1920s, though production peaked at about 2 million barrels a day in the early 1970s. For decades, geologists have known that oil could be found in different layers of rock piled up like a stack of geologic pancakes. But now drillers are starting to tap those layers simultaneously from a single site—and are committing billions of dollars to do so. […]