942 EXPLORATION GEOPHYSICS 



concentrations of hydrocarbons occur in the soil air over productive zones, 

 with relatively lower values over barren areas. 



Figure 578 illustrates, ideally, the significant hydrocarbon distribution 

 patterns as reported by Russian workers. High concentrations of methane 

 are observed in the soil air over gas accumulations, while high concentra- 

 tions of the heavy fraction are noted over the oil accumulations. Low 

 concentrations are found in the soil gas over barren areas. 



Theoretical Interpretation of Gas Analysis Anomalies. — The development of 

 interpretative technique by the Russian investigators has proceeded along the lines 

 of attempting: (a) to deduce theoretical curves characteristic of petroliferous and 

 gaseous deposits at various depths below the surface and (b) to solve the inverse 

 problem : viz., infer the depth of the deposits from the observed anomaly curve. 



Antonov t has published an article in which he assumes that diffusion takes place 

 through a homogeneous overburden. In the stationary state, the diffusion equation 

 reduces to Laplace's equation which may be written in the form : 



3.r^ 'dy^ "bz" 



where M is the concentration or partial pressure of the hydrocarbon gas. Typical 

 boundary conditions used by Antonov are: (a) the region below the source is com- 

 pletely impervious and (b) no diffusion takes place through the surface of the earth 

 into the air. The validity of such boundary conditions is not self-evident. 



In addition to this theoretical technique, the Russian investigators employ various 

 empirical techniques derived from extensive investigations over many oil and gas fields. 

 For example, investigations of various deposits have shown that the percentage of 

 heavy hydrocarbons increases over an oil horizon and the percentage of methane 

 generally increases over a gas deposit.^ The ratio of the light to the heavy hydrocarbon 

 content is assumed therefore to furnish a useful diagnostic variable. 



The Emanometric Method. — Another technique, known as the 

 emanometric or geodynamic method, should be classified with gas analysis 

 methods. This technique§ endeavors to measure the absolute rate of ema- 

 nation per unit area of the earth surface of one or more diagnostic gases 

 and to recognize a commercial oil or gas accumulation by near-surface 

 measurements. Ethane is considered the most diagnostic constituent and, 

 over a square foot of area, the emanation of this gas is calculated to be 5.2 

 cu. mm. per 24 hours measured under normal conditions of temperature 

 and pressure. 



In conducting a survey, selective adsorbent tubes are placed over a 

 measured and confined surface of the ground for a specified period of time, 

 usually of the order of twenty-four hours. After the exposure time is 

 completed, the adsorber tubes or "emanometers" are sealed and taken to 

 the laboratory for the next steps which include degassing, fractionation, 

 distillation, identification, and quantitative measurement of the diagnostic 

 gases. After correcting the data for topography and ground water percola- 

 tion, there remains the anomaly map, which is the leakage configuration 



t P. L. Antonov, "Contribution to the Theory of Gas Surveying," Neft. Khoz. 26, No. 5, 

 pp. 19-23 (1934). 



X B. N. VictorofF, "Interpretation of the Nature of a Gas Survey," Neft. Khoz., Sept., 1934. 



§ Sylvain J. Pirson, "Emanometric Oil and Gas Prospecting," The Petroleum Engineer, Janu- 

 ary, 1946. 



