A system for the quantitative analysis of one or more evolving gases exiting a borehole includes a Venturi ejector for substantially capturing liberated gases in the bell nipple and return line, a rotating disk extractor for substantially extracting gases entrained and dissolved in drilling mud, and a gas analyzer system for analyzing and quantifying the captured and extracted gases. The Venturi ejector preferably is operatively coupled to the return line, and a pipe wiper may be arranged to partially cover the bell nipple. The ejector sucks liberated gases out of the return line and causes a negative pressure to occur at the bell nipple such that air is sucked into the bell nipple rather than exiting the same. The rotating disk extractor has air flowing countercurrent to the mud flow. As the disks rotate, they pick up a thin liquid film of mud which is exposed to the air stream. Dissolved hydrocarbon gas in the thin film evaporates, and entrained bubbles break resulting in the gases entering the air stream which is passed through a liquid trap out to a gas analyzer. By knowing the rate of air flow through the ejector and the extractor, the rate of mud flow through the system and through the extractor, and the gas compositions as determined by the gas analyzer, the quantitative components of the different gases in the mud are found. Also, with known rates, the gas flows may be combined in a proper ratio to provide a single representative gas stream.