Chap. 12] MISCELLANEOUS GEOPHYSICAL METHODS 907 



geologic structure and nature of hydrocarbons mapped; besides, there is 

 confusion in some of the recent articles in regard to geologic significance 

 of the earlier data on the subject. The claim is certainly unjustified that 

 in regard to halos, different results are obtained by the free-gas analysis 

 (using the Sokolov or Laubmeyer techniques) and the soil analysis (map- 

 ping adsorbed and entrained constituents). 



Laubmeyer definitely states in his publication that a sharp drop above 

 production was observed in the Oberg field and attempts to explain the 

 phenomenon by the withdrawal of gas from the productive formations by 

 the wells in the center of the field. A survey of the Nienhagen field*^ re- 

 vealed a maximum gas concentration above the western part of the field, 

 while the eastern portion shows nearly normal concentration. On the 

 north side of the Wietze dome^' the gas maximum is forced northward or 

 away from the productive zone, and the same trend is indicated by the 

 gas measurements on the Pierce Junction dome (see Fig. 12-lla). It may 

 be argued that in the vicinity of salt domes, the indications are related to 

 fracture zones; however, this still leaves it unexplained why the indications 

 should be forced consistently outward and away from them. On the other 

 hand, this argument shows the necessity for considering closely the geologic 

 structure associated with a halo. It is evident that a halo around a salt 

 dome is not directly comparable with a halo around production above a 

 stratigraphic trap because in the former we deal with steeply dipping beds 

 around an impervious core (unless production occurs above the dome) 

 while in the latter any evidence of folding or faulting is absent. Mac- 

 roscopic halos have been reported for several Gulf coast fields,^* such as 

 Goose Creek, Humble, and Sour Lake. 



The true halos, then, may be assumed to occur in regions of gentle 

 folding and above stratigraphic traps. Thej'^ have been variously ex- 

 plained, the prevalent assumption being that some sort of clogging occurs 

 near the center of the productive structure. McDermott^^ believes that 

 because of the tendency of oil and gas to migrate to the highest point of a 

 fold, monocline, or lenticular deposit, these become clogged first, the seal- 

 ing action working its way down the dip. This clogging is not noticeable 

 at the surface if subsequent folding has developed cracks in the caprock 

 and thus provided an avenue of escape for the gases near the top. The 

 surface effect is thus similar to that of a buried fault (linear source). 

 Rosaire, on the other hand, assumes that such clogging occurs fairly near 



*^ G. Laubmeyer, unpublished reports. 



6^ Rosaire, Geophysics, 111(2), 107 (March, 1938). 



« Geophysics, IV(3), 1-15 (July, 1939). 



** Handbook of Geochemical Prospecting, op. cit., p. 22. 



