often possible to apply with considerable accuracy the knowledge gained from 

 logging one or more wells in which the formation properties are known. In 

 this manner liquid-filled porosity can be determined quantitatively and other 

 formation characteristics, such as shale content or uranium content, can be 

 estimated. To determine porosities from the neutron curve, one first establishes 

 a relationship for the particular field by comparing the core-analysis data with 

 the neutron log of a representative well. In general, the neutron-curve response 

 diminishes in an approximately logarithmic manner as the porosity increases. 

 As illustrated in Figure 15-5, points are plotted on semilogarithmic graph paper, 

 core porosities along the logarithmic axis, and neutron curve deflections along 

 the linear axis. The best straight line then is fitted to the points. Porosities in 

 other similar nearby wells are then read from this graph according to the 

 neutron-curve deflections recorded therein. This method is particularly effective 

 in clean (nonshaly) limestone reservoirs. 



In the absence of reliable core data, it is still possible in many instances 

 to estimate porosities from the radioactivity log. Familiarity with local con- 

 ditions often makes it possible to state with assurance that the porosity of a 

 particular shale-free layer is less than 1 or 2 percent and that the porosity 

 required to diminish the neutron-curve response to the intensity observed in 

 shale is, say, 40 percent. In this way two points are obtained between which 

 a logarithmic scale of porosities may be marked off. 



Shale and uranium content may be estimated from the gamma-ray curve 

 by comparing (on a linear basis) its response in the zone of interest with the 

 response obtained in a known shale or a zone of known uranium content. For 

 example, a response one fifth as great as that of a nearby shale may be taken as 

 indicative of 20-percent shale content. 



In many instances a strict quantitative relationship has been found to 

 hold between the gamma-ray log deflection and permeability within a particular 

 formation because of the fact that the shale content, which is related to the 

 permeability of the rock, is indicated by the curve. In other instances this 

 indication of the degree of shaliness appears to correlate better with formation 

 porosity than with permeability. 



APPLICATIONS OF Radioactivity logs are run most frequently 



RADIOACTIVITY LOGGING at the time of completing the well, either just 



before or just after setting casing. At this 

 time the log is employed to determine the depth and thickness of the various 

 strata, to identify them, and to correlate their depths from well to well in order 

 to establish their structural position. Other data which the radioactivity log may 

 be called upon to provide at this stage include: position of zones of maximum 

 porosity and fluid content, depth of the gas-oil contact, position of the fluid 



338 



