reservoir conditions limits the accuracy of the estimate of the formation'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 mi- 

 gration 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 18-1, the less-permeable character of the bed from 3669 feet to 3673 feet 

 was indicated both by drilling-time and electric logs. 



There are many cases where thin shale breaks in a sandstone reservoir 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 to determine the stratigraphic and 

 structural position of a well which might otherwise remain unknown. It may 

 also reveal the presence of probable porous beds that may be saturated because of 

 their structural position, and therefore the economic risk of drilling a new hole 

 or abandoning a location may be substantially reduced. 



Since the publication of the first edition of this symposium, the writer has 

 received communication from J. A. Simons, geologist with Creole Petroleum 

 Corporation, regarding the use of drilling-time and drilling-rate curves in 

 Venezuela. He states, "This technique (drilling-time logging) is extremely useful 



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