The shale-oil tsunami that has flooded New Mexico with a wave of petro dollars in recent years may be receding somewhat as U.S. production faces its first significant decline since the gushers began nearly eight years ago. Signs of trouble are showing up in major basins around the country, with a significant drop in drilling rig activity and sluggish growth this year compared with the spectacular bursts of production in recent years that has transformed the U.S. into the world’s No. 1 oil producer.