Subsurface Logging Methods 465 



standing physical and chemical reservoir conditions and factors relating 

 to the recovery of petroleum resources, the amount of reserves in place 

 is given only as an estimate. Some even discount the value of making 

 estimates of this character because of the lack of knowledge or possession 

 of empirical data necessary to arrive at reliable conclusions. Efforts are 

 continuously being made to increase the accuracy of reserves estimates. 

 The oil or gas content of a reservoir bed is generally given in barrels of oil 

 or mcf. of gas per acre foot. Lack of adequate knowledge pertaining to 

 the reservoir conditions limits the accuracy of the estimate of the for- 

 mation's content per unit volume, but the factor given is the best available 

 in light of present-day scientific understanding. An estimate of the areal 

 extent of a reservoir bed is also subject to considerable latitude because of 

 the lack of knowledge concerning migration channels and underground 

 drainage conditions. The estimate again is made on the best information 

 that can be supplied by the subsurface geologist after mapping the structure 

 in which the producing horizon is found. 



In establishing the net effective thickness of the reservoir bed, there 

 is a greater means of eliminating the necessity of an estimate, provided 

 adequate data are available. In a section where reservoir beds are very 

 uniform in respect to lithology, porosity, and permeability, the thickness 

 may be determined by the driller's record, an electric log, a radioactivity 

 log, or other reliable methods of logging or observing a formation. In 

 those reservoir beds where the lithology is not uniform all available means 

 may be required to determine the net effective thickness of that bed. Core 

 information and analyses, electric and radioactivity logs, and drilling-time 

 logs contribute to the best possible answer. As shown in figure 215, the 

 less-permeable character of the bed from 3,669 feet to 3,673 feet was in- 

 dicated both by drilling-time and electric logs. 



There are many cases where thin shale breaks in a sandstone reser- 

 voir or tight calcareous streaks interbedded with saturated sandstone are 

 not indicated on electric or radioactivity logs. If recorded on a short 

 enough depth interval, however, drilling time will seldom fail to disclose 

 the presence and thickness of such breaks. The writer has used drilling 

 time recorded at intervals of one-tenth of a foot in a productive section 

 where many and very thin impervious streaks were present and only by 

 this means was able to determine the exact net effective thickness of the 

 reservoir. Therefore, if positive information can be gained as to the 

 thickness of a formation, one of the three essential data making up a 

 reserves estimate can be assigned a fixed value, and the accuracy of the 

 final answer is increased. 



Perhaps the greatest argument for the use of drilling-time logs is their 

 value as insurance against the loss of geologic information in the event 

 that other types of logging are precluded because of well conditions or in 

 case of a junked hole. Because a drilling-time log provides essential data 

 corresponding to an electric-potential log, it can be used for correlation 



