Chap. 12] MISCELLANEOUS GEOPHYSICAL METHODS 887 



waters may originate from the leaching of salt deposits. Further, active 

 seepages indicate a depletion of a subsurface reservoir and there is always 

 doubt as to whether it is in its initial or final stages. Conversely, many of 

 the best sealed (mid-continent. Paleozoic) fields have no seepage indica- 

 tions at all. Even when they are accompanied by structural and strati- 

 graphic observations, the use of surface indications has the obvious limita- 

 tion that it is impossible to determine the length and direction of the 

 supply channel from the visible surface concentrations. The surface evi- 

 dence may be far removed from a commercial subsurface accumulation. 

 In this respect, microscopic methods of observation and analysis are more 



Table 82 

 DETECTION OF HYDROCARBONS AND ASSOCIATED INORGANIC 



COMPOUNDS 

 I. Macroscopic Methods 



A. Direct Indications 



1. Oil seepages 



2. Gas emanations 



3. Asphalt deposits 



4. Oil impregnations 



5. Wax (ozokerite) 



B. Indirect Indications 



1. Salt, sulfur 



2. Bromine and iodine waters 



3. Gas mounds 



4. Mud volcanoes 



5. Sandstone and clay-breccia dikes 

 II. Microscopic Methods 



A. Gas Detection 



(volatile constituents in interstitial soil air) 



B. Soil Analysis for Occluded Constituents 



1. Volatilizable fraction 



Hydrogen, methane, ethane, propane, butane 



2. Extractable fraction 



Organic liquids, waxes, inorganic solids 



successful, since they locate not only concentrations on outcropping 

 fissures, faults, and the like, but also accumulations resulting from the 

 continuous flow by diffusion from the reservoir through the overlying 

 strata. The pattern of indications furnished by the microscopic methods 

 is therefore more uniform and has been found to show certain character- 

 istic patterns in the fields thus far investigated. 



The micro methods fall into two groups: gas detection and soil analysis. 

 In the former the concentration of gases in the interstitial soil air is deter- 

 mined by withdrawing it from shallow holes and passing it through a 

 portable detector, usually of the "hot wire" type. Soil-analysis methods, 

 on the other hand, require an analysis of soil samples for their content of 

 hydrocarbons and other significant constituents. There are two main pro- 



