Oasis Petroleum Inc, the last major oil producer operating exclusively in North Dakota’s Bakken shale formation, is heading south to the nation’s largest oilfield, pursuing a pricey expansion as investors worry about maturing shale fields. Oasis shares have tumbled 21 percent following last week’s disclosure, pointing to growing tension between oil producers looking to invest in their future and investors demanding better returns. Oasis said the $946 million cash-and-stock investment secures a premier foothold in the Delaware basin area of the Permian, the nation’s fastest-growing oil field, while spreading its costs over a wider drilling footprint. The Bakken, one of the first U.S. shale fields to be tapped, is maturing, oil analysts say, noting that by some measures it has just five or six years of sweet-spot drilling sites left in its core. “Oasis […]