Across oil fields from Texas to North Dakota fears are growing that crude’s plunge below $30 a barrel is more than just another market milestone and marks a countdown to an endgame for many shale producers that so far have braved the 18-month downturn. Oil prices tumbled by more than a fifth this month to 12-year lows 70 percent below mid-2014 levels and traders brace for more declines as world production keeps outpacing demand. It’s bad everywhere,” said Raymond Lasseigne, president of privately-held TMR Exploration in Bossier City, Louisiana.