Apache Corp. said it has discovered the equivalent of at least two billion barrels of oil in a new West Texas field that has the promise to become one of the biggest energy finds of the past decade.

The discovery, which Apache is calling “Alpine High,” is in an area near the Davis Mountains that had been overlooked by geologists and engineers, who believed it would be a poor fit for hydraulic fracturing. It could be worth $8 billion by conservative estimates, or even 10 times more, according to the company. Shares rose by...