CHEMICAL METHODS 943 



believed to be due to confined pressure sources of oil and gas pools, but 

 which must be subjected to further reduction for climatic, seasonal, and 

 atmospheric effects ; for nature of soils and in particular for soil perme- 

 ability; and finally, for deep geologic effects. Once the reduction of the 

 various influences have been taken care of, the residual map is the anomaly 

 map representing the diffusion zone from oil or gas fields. 



Soil Analysis Methods 



The development of soil analysis techniques is due primarily to Amer- 

 ican investigators. These methods may be classified into: (1) those tech- 

 niques which involve the extraction and measurement of the volatile, 

 saturated hydrocarbons in the soil; and (2) those which involve the deter- 

 mination of one or more constituents that are present in the soil in larger 

 than normal quantities through secondary enrichment. Among the materials 

 determined are organic wax-like and liquid materials, fluorescent materials, 

 and inorganic compounds such as carbonates, sulfates, and chlorides. 



Soil Analysis for Volatile Hydrocarbons. — The modification of the 

 Russian technique was introduced by Rosaire and Horvitz who began 

 their investigations in 1936. This approach is based on the thought that 

 during the upward migration of the hydrocarbons from the oil deposit, 

 there is present a strong tendency for their adsorption and occlusion by the 

 soil particles. It was anticipated that analysis of the soil itself for these 

 entrained constituents would yield substantially greater concentrations 

 than would result from the analysis of the interstitial air. 



While the soil gas methods depend upon a large and undetermined 

 quantity of soil from the interstices of which the air sample is drawn, the 

 soil analysis method has apparently yielded satisfactory results with samples 

 as small as fifty grams. f In the collection of gas samples in the field by 

 the methods previously described, insurmountable difficulties are encoun- 

 tered which limit proper sampling to restricted areas. For example, satis- 

 factory soil air samples cannot be obtained in situ from cohesive clays, 

 in alluvial gravel deposits, in sand hills because of intensive aeration, in 

 regions with a high ground water level. $ 



Sampling. — Sample locations are first surveyed over the area to be 

 investigated (Figure 579). Care must be taken to locate the stations at 

 considerable distances from roads, pipe-lines, drilling wells, and other 

 sources of possible contamination. The bore hole may be dug with bucket- 

 type hand augers, or with mechanical drilling equipment. In either case, 

 lubricants should be avoided. When the desired depth is reached, a sample 

 is brought to the surface, placed in a pint glass jar, and securely sealed 

 (Figure 580). The sample jars are carefully labeled as to station number, 



t Leo Horvitz, "On Geochemical Prospecting," Geophysics, Vol. 4, No. 3, pp. 210-225, July, 

 1939 



t V. A. Sokoloff, Monograph published in U.S.S.R., 1932. 



