Delineation of Formations and Determination of Their Boundaries 



The conventional resistivity curves (chiefly the short normal and the SP 

 curve, usually give sufficient data for correlation of unconsolidated and moder- 

 ately consolidated formations (medium- and high-porosity sands and sand- 

 stones and shales) . An example of correlation is given in Figure 14-22. The 

 SP curve is fundamental for the delineation of permeable beds. In addition, 

 the MicroLog, run preferably with a microcaliper, is necessary for a more 

 accurate definition of the boundaries, particularly those of thin shale beds and 

 hard streaks interbedded with the permeable sections. The MicroLaterolog 

 may be used instead of the MicroLog. In very salty mud, the SP curve is 

 replaced by the gamma ray curve. 



The conventional resistivity curves (normal and lateral) and the SP curve 

 usually delineate large bodies of formations with different average lithological 

 characteristics and make possible good general correlations of hard formations 

 (limestones, dolomites, and highly consolidated sandstones) . The curves very 

 often do not give an accurate representation of thin sections with different 

 lithologies. The limestone curve reflects more closely the lithologic variations 

 and helps for correlation. The Laterolog gives a more detailed and accurate 

 record of the formations and makes possible better correlation. The most 

 detailed and accurate record of porous and permeable beds, of tight sections, 

 and of shales is given by the MicroLog or the MicroLaterolog with the help of 

 the SP curve, the microcaliper, and the gamma-ray curve. The radioactivity 

 methods (gamma ray and neutron), although they do not give as much informa- 

 tion as the electric methods, are good tools for correlation. 



Investigation of Porosity and Fluid Content 



Evaluation of Porosity 



Unconsolidated and Moderately Consolidated Formations 



The determination of porosity from the electric logs is based essentially 

 on the measurement of R^ by MicroLog or the MicroLaterolog, as seen 

 previously. A fair accuracy in the evaluation of porosity (down to porosity 

 values of 15 percent) is obtained with the most recent type of MicroLog 

 equipment. 



Another approach, which can be used to check the results of the MicroLog 

 or to replace the MicroLog in case it is not available, consists of taking the 

 formation factor equal to R /R w , R being the true resistivity of the formation 

 at a level where it is 100 percent water-bearing. Of course, the extrapolation 

 of the value of F thus determined to the oil-bearing section within the same 

 formation implied that the lithological characteristics are approximately con- 

 stant, which is not always the case. 



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