U.S. oil and gas producers Diamondback Energy (FANG.O) and ConocoPhillips (COP.N) said on Tuesday the top U.S. shale field will face natural gas pipeline constraints in coming years as production continues to grow. While other U.S. oil basins are seeing production plateau, the Permian Basin in west Texas and New Mexico is anticipated to continue to grow because of its low cost of output. At the same time, operators there are under pressure to reduce the flaring of excess natural gas, they said.