Gas container

A container can be seen behind two Heretic Grunts.

The Forerunner's mining facilities use Gas Containers to move the volatile gases from location to location within the facility. They can explode if exposed to weapon fire or an explosion in the immediate vicinity. You can use these to kill Heretic forces on the level The Arbiter. The also appear on the Multiplayer level Colossus. The gas inside these containers is most likely methane because if your not careful it can explode and it looks to be the same gas inside the Grunts rebreathers.

The gas could also be a possible fuel used to power the station and the containment shields of the Flood perimeter.