Harold Hamm, the wildcatter who helped usher in the American fracking boom, has weathered his share of oil busts. None of them matched this one. The 13th child of Oklahoma sharecroppers, Mr. Hamm rose from the bottom of the oil business to become a self-made billionaire. He is one of the pioneering prospectors who turned the U.S. into the world’s leading oil producer by using hydraulic fracturing and horizontal drilling techniques to unlock huge volumes from rock formations. …