Most investors can tell you how much a barrel of oil fetches today. Those in the industry can, too, of course, but they are watching another number closely: how much it costs to produce that barrel. While that varies geographically and geologically, it has become cheaper nearly everywhere. And that has an impact on where the production declines will occur that are needed to balance oil supply and demand. In North America, for example, where relatively expensive but abundant shale oil fields transformed the market in recent years, costs are down sharply . Deutsche Bank DB -1.90 % oil-field-services analyst Mike Urban estimates that exploration-and-production companies have cut well costs by 30% to 50% in that time—but that half to two-thirds of that is “cyclical.” In other words, part is due to technical advances […]