Figure 28-4. Analyses of the soil for entrained hydrocarbons shows the anomaly maximum 

 to fall over the edges of the oil accumulation (Taylor) . 



termined by burning over a glowing platinum wire and then measuring the 

 resulting carbon dioxide. Ethane and the heavier hydrocarbons are determined 

 by first vaporizing and then measuring the volume of the condensed fraction. 

 This fraction then is ignited, and the resulting carbon dioxide in the final volume 

 is a measure of the quantity of ethane and heavier hydrocarbons initially pres- 

 ent. Sensitive McLeod gauges are used to indicate the pressures from which the 

 volume of hydrocarbons are calculated. After these volumes are converted to 

 weights, on a dry basis, they are expressed finally in parts-per-billion of dry 

 weight of the soil sample. A photo of the Horvitz laboratory setup is shown 

 in Figure 28-2. The precision of this method is claimed to be within limits of 

 20 percent for values exceeding 10 parts-per-billion. Costs of a soil survey, 

 according to Horvitz, are generally about one fourth that of a seismic survey. 



An example of a soil survey showing ethane-propane values collected over 

 a sand lens is shown in Figure 28-3. Note that the producing zone is limited to 

 the low ethane-propane values which are bordered by higher concentrations. 



Figure 28-4, a simplified drawing of a typical hydrocarbon anomaly, shows 

 a maximum concentration around the edges of the oil accumulation. This differs 

 from the halo obtained by soil-gas techniques (fig. 28-1), wherein the methane 

 appears highest directly over the pool of oil. Rosaire (1938) explains these 

 differences by assuming that the strata overlying the oil pool are made less 

 permeable to migrating hydrocarbons because of the deposition of minerals from 



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