Chevron’s existing assets give the company a very different calculus than newcomers would face. But the timing could not be worse: Global crude oil prices have steadily declined over the last several years, recently dropping below $60 a barrel — approaching the break-even point for many American operators. That’s been driven by global supply surpluses and by weakening demand, as renewable energy prices drop. “I think what we’re seeing is that the days of the oil and gas industry being the growth engine of economies is well behind us,” said Trey Cowan, an oil and gas energy analyst at the Institute for Energy Economics and Financial Analysis.