The critical desorption pressure is determined by pumping out an area within a well bore. As the pressure in that area is decreased, formation fluid from the surrounding rock matrix is drawn into the area. As the local pressure is decreased further, gaseous bubbles within the formation fluid will begin to form. The formation fluid is observed, and the formation of the gaseous bubbles is detected. The critical desorption pressure is determined based upon the presence of gas within the formation fluid. The concentration of gas within the formation fluid can then be determined based on the determination of the critical desorption pressure.