Background Gas
Background gas expresses the average gas reading that is observed while drilling. It is also sometimes called "Drill Gas". It is an indication that the formation being drilled contains gas but doesn't necessarily an indication of higher formation pressure. The gas enters the wellbore only when a new formation is drilled. It will seep in from the already drilled part of the well only if the formation has enough permeability and pressure differential. Background gas doesn't indicate an underbalanced condition but may lead to it if the total volume of gas in the system lightens the hydrostatic column enough.