BORE-HOLE INVESTIGATIONS 1103 



It may be observed from this table that the average intensity values of these 

 formations (approximately the midpoints of the ranges) permits subdivision of the 

 formations into three general groups : 



Group 1 — Formations Having Low Radioactivity- 

 Sandstone, limestone, dolomite, salt and anhydrite come under this category. 

 In general, sandstone, limestone, and dolomite show similar, low-intensity radioactivity 

 values and cannot be reliably distinguished from each other by their radioactivity 

 alone. Some knowledge of local stratigraphy is therefore necessary for their reliable 

 identification. Mixtures of limestone, dolomite, and pure sandstone are about as weak 

 radioactively as the pure types. Porous and non-porous sandstone, limestone, and 

 dolomite exhibit the same degree of radioactivity. There appears to be little, if any, 

 difference between the radioactivity values of uncemented sands and those showing 

 various degrees of cementation. The presence of interstitial water in formations 

 appears in general to have no measurable influence on their radioactivity. 



Group 2 — Formations Having High Radioactivity- 

 Shale, clay, and silt are the most common formations having a normally high 

 order of radioactivity. Whenever these materials either contaminate, or are highly 

 interbedded with, formations having low radioactivity, they increase the radioactivity 

 of the zone. Thus mixtures such as shaly sand, shaly limestone, sand, shale, calcareous 

 shale, etc., commonly show medium to high values of radioactivity. 



Group 3 — Formations Having Abnormally High Radioactivity 



Formations possessing abnormally high values of radioactivity are black or 

 organic marine shale, granite, bentonite, and volcanic ash. Igneous rocks in general 

 possess very high radioactivity values. Formations in the lower radioactive groups 

 are frequently contaminated with bentonite and volcanic ash with the result that they 

 show increased radioactivity. Some underground waters are very highly radioactive 

 and are capable of making mineral deposits which are very radioactive. Such ground 

 waters rising through fault planes or fractures, or traveling through unconformities 

 and making such deposits may cause distinct radioactive markers. (See Figure 627, 

 Qiapter X.) 



Typical Response of Formations 



Figure 689 is a chart which will aid in the general interpretation of radioactivity 

 logs. The gamma-ray curve, on the left, represents typical response to a wide variety 

 of formations. The following description is an elaboration of the brief description 

 indicated on the figure. 



Surface Formations. — The susceptibility of the gamma-ray curve to cosmic 

 radiation and the gamma radiation from soil is indicated at the top of the curve. 

 Although beds capable of producing oil or gas rarely occur close enough to the surface 

 to involve the effect of cosmic rays, it is worthy of note for the few instances in 

 which interpretation to the surface is desired. 



Uncontaminated Formations. — The typical response of uncontaminated sand, 

 lime, dolomite, salt and anhydrite is illustrated at various points on the graph. Such 

 formations are characterized by low, relatively uniform radioactivity. Typical response 

 patterns for relatively pure limestone and dolomite appear to be the most highly de- 

 veloped in practically all areas in which wells have been surveyed. Salt and anhydrite 

 are normally very low in radioactivity and are frequently quite easily distinguished 

 from other formations. Normal shales show reasonably constant radioactivity values 

 throughout the log. 



Contaminated or Interbedded Formations. — As the amount of shale, silt, etc., 

 intermixed with sand, lime or dolomite increases, the gamma-ray curve shows in- 



