The recent nosedive in crude oil prices came just as shale producers had started delivering healthy returns after years of heavy spending to boost production and market share.  The shift has pleased investors who had grown weary of waiting for a payoff while watching the frenetic west Texas shale boom make the United States the world’s top oil producer and a major exporter. The 29 percent drop in U.S. crude oil prices CLc1 since October now threatens those improved margins, and sustained prices below $50 per barrel could dent the value of shale reserves, which banks use to determine borrowing power. Activity in the largest U.S. oil field could fall 10 to 20 percent next year if prices […]