498 



Subsurface Geologic Methods 



applying a vacuum to the container, the air-gas mixture is drawn over a 

 hot platinum filament of a balanced electrical circuit. If any combustible 

 gases are present in the mixture, they will increase the temperature of the 

 platinum filament and consequently throw the system out of balance. 

 This differential is recorded on a galvanometer. Methane and ethane ignite 

 at a temperature considerably higher than propane, butane, etc., and shale 

 gas. Thus it is possible to distinguish readily between the higher- and 

 lower-burning-point gases. The temperature of the platinum filament is ad- 



FiGURE 234. Interior view showing a sample agitator with gas analyzer above. Infrared 

 sample drier can be seen at left and mud-testing equipment at right. 



justed to ignite all hydrocarbon gases present and is then lowered to a 

 point at which the methane and ethane will not ignite. The difference be- 

 tween the two readings will thus be the reading of the methane and ethane 

 present. As this is a qualitative test only, results must be correlated with 

 the lithologic characteristics of the section, for porosity and permeability 

 control the magnitude of the readings obtained. 



Petroleum oils show a fluorescence under an ultraviolet light. The 

 maximum fluorescence occurs with a lamp having a wave length of be- 

 tween 3,200 and 3,800 angstroms. A 230-volt mercury-vapor lamp with 

 suitable filters is considered most effective for detecting residual oil in 

 the cuttings. The lamp is enclosed in a light-tight box with a viewer that 

 has a magnification of approximately 3| times. Samples are examined 



