Abstract: An oil well drilling rig (10) recirculates drilling mud which is analyzed continuously by pumping it through a gas separation unit (72) where the gases in the mud become separated and mixed with a carrier gas and are conveyed to gas analyzing devices (196) where the concentration of the different hydrocarbon components of the gases in the mud are continuously measured and the signals representative of these measurements are processed together with mud flow rate, and the carrier gas flow rate or the sample gas flow rate signals in a signal processor (64) to provide a continuous log of gas component concentration during drilling.