CHEMICAL METHODS 949 



trations at the edges of the producing areas. f One or two cases were noted 

 by these workers^ but explained away to their satisfaction, leaving their 

 original conclusions unchanged. 



One explanation of the observed soil survey patterns has been proposed 

 by Rosaire§ who suggests that the "halo" pattern results from shallow 

 mineralization produced by the deposition of minerals from ground waters 

 during their upward migration. This mineralization produces a less per- 

 meable zone immediately over the structure, so that subsequent migration 

 of hydrocarbons would result in relatively greater concentrations around 

 the edges, rather than over the center. He substantiates his view by point- 

 ing out that drilling rates are lower for wells over structures than for 

 those in the immediately surrounding territory. The existence of electricalf f 

 and refraction anomalies over structures further supports his theory. 



The application of soil analysis to stratigraphic prospecting is illus- 

 trated in Figure 583, which shows the ethane and propane values obtained 

 by analysis of subsurface samples collected over a producing sand lens 

 located in South Texas. 



An example of the type of data|| obtained by the acid extraction tech- 

 nique is shown in Figure 584, which gives the results of a reconnaissance 

 survey over the Heidelberg area, Jasper County, Mississippi. In this survey 

 analysis was made of 364 samples collected at depths of 12 feet. The station 

 locations together with the total hydrocarbon values (methane plus ethane 

 and heavier hydrocarbons) are shown on the map. The field development 

 at the time the survey was started is shown by the well symbols. In order 

 to ascertain the background values, data were obtained over an area extend- 

 ing four to six miles east, west, and south of the then known producing 

 limits. 



The following groups of values (expressed in parts per billion by 

 weight) are enclosed by the various contour lines: 50-99; 100-199; 200- 

 399; 400-799; and 800 and above. In order to display the variations in 

 hydrocarbon concentration over the area sampled, various degrees of 

 shading have been used ; the darker the shading, the higher the concentra- 

 tion. Values below 50 parts per billion by weight are in the unshaded areas ; 

 values from 50-99 parts per billion are in the areas with the lightest shading ; 

 values from 100-199 parts per billion are in the medium shaded areas ; 

 and values of 200 parts per billion and above are in the zones with the 

 heaviest shading. 



The areas of low hydrocarbon concentration which are bordered by 

 relatively high values are considered by Horvitz to be the favorable areas. 

 The outer limits of the anomalous area are marked by the band of high 

 hydrocarbon concentration which starts in Section 21, T. 1 N., R. 13 E., 



t E. E. Rosaire, Geophysics, Vol. 4, No. 4, pp. 300-304, 1939. 



% V. A. Sokoloff, Neftyanoye Khozyaystvo, Vol. 27, No. 5, pp. 28-34, 1935. 



§ E. E. Rosaire, "Shallow Stratigraphic Variations over Gulf Coast Structures," Geophysics, 

 Vol. 3, No. 3, pp. 96-115, 1938. 

 tt See Figures 299, 351, 352. 

 tX Leo Horvitz, Geopliysics, Vol. 10, No. 4, pp. 487, 493, 1945. 



