Abstract: A detection unit (20) for detecting hydrocarbon gases includes a cylindrical housing (20) having a pair of opposed rectangular slots (56, 58) formed in the wall of the housing near the lower end (52). A support (60) is mounted within the housing (50) adjacent the upper end (54). A pair of baffle plates (62, 64) are mounted within the housing from the walls thereof. A semiconductor hydrocarbon gas detector (61) is mounted to the support (60) and is connected by electrical leads (30) to an analyzer and recorder (28). The detection unit (20) is mounted at the mouth of an oil or gas drilling rig and receives drilling mud into the housing (50) at the rectangular slots (56, 58) such that gases released from the drilling mud are analyzed by the gas detector (61) to determine the hydrocarbon gas content therein.