Ch. 31] CORES 569 



meter, pump-stroke counter, and pump-rate meter. The drilling rate 

 is shown in feet per hour. Direct correlations between depth positions 

 of sand cuttings, increased rates of drilling, and depth positions of 

 oil and gas are immediately apparent from such a log. This type of 

 combination log has much value in the drilling of wildcat or explora- 

 tory wells. Continuous mud and sample logging affords a direct basis 

 for deciding which formations in a wildcat well should be cored. In 

 the Permian basin of west Texas it is difficult to obtain good electric 

 logs owing to such factors as thick limestone sections and high saline 

 content of the drilling muds. Mud and cutting analysis logging af- 

 fords a means of supplementing the log data that can be obtained. 



CORES 



The ideal source of information from a well is a core of the forma- 

 tions penetrated. To core a well is an expensive procedure, and for this 

 reason coring usually is restricted to the producing zones or forma- 

 tions that may be productive of oil or gas. Coring has received con- 

 siderable impetus in the past few years by the general use of the dia- 

 mond bit. With it more complete recoveries of the softer formations 

 are obtained, and in hard formations it is sometimes actually cheaper 

 to diamond-core than to drill with the conventional bit. For many 

 years water-base muds have been used, but in the past several years 

 petroleum engineers have been turning more to the use of oil-base 

 muds for coring. Both types of mud partially flush out the original 

 fluids of the core. Oil-base mud, however, is more practical when oil 

 pay sections are being cored above the water zone in reservoir sands. 

 The water content of such cores is bound to be more nearly representa- 

 tive of the original water in place than would be the case if water base 

 mud were used. 



As soon as a core is taken from the core barrel, it is washed and 

 described. The examination may be aided by the use of a binocular 

 microscope. It is next taken to the laboratory, where it is tested by 

 the reservoir engineer for porosity, permeability, oil and water con- 

 tent, capillary pressure, etc. Figure 5 is an example of the plotted 

 description of a producing formation with the results of porosity and 

 permeability determinations restricted to that portion of the cored 

 interval capable of producing oil and gas. Some of the properties of a 

 reservoir sand are discussed in the section on permeability in Chapter 

 32. Cores, rather than sample cuttings, are essential for such detailed 

 studies. The geologist uses core samples principally for studies of 

 texture of clastic grains, microfossils, qualitative and quantitative 



