The SP curve and the short normal curve give a good record of the 

 boundaries of the shale and sand beds provided they are thick enough. The 

 very thin breaks within sand bodies are shown only by minor changes on the 

 curves. 



Moreover, very thin hard beds in shale formations, which are rather 

 poorly indicated by the SP logs, show up sufficiently clear on the short normal 

 curve recorded with an amplified scale to be useful as markers for correlations. 



Generally, the resistivity of the mud is much greater than that of the 

 connate water; on the Gulf Coast, for example, the mud resistivities at forma- 

 tion temperature are confined to a range from about 0.5 to 2 ohm-m, whereas 

 the resistivities of the connate water are generally lower than 0.05 ohm-m. 

 The resistivity of the invaded zone is, therefore, much higher than the true 

 resistivity of the water-bearing formations and often exceeds the true resis- 

 tivity of the oil-bearing formations. The apparent resistivity measured with 

 the short normal is greatly affected by the invaded zone; and peaks are 

 generally obtained on the log in front of the permeable beds, even if their true 

 resistivity is very low. Sometimes, however, the depth of penetration of the 

 mud into the permeable layers is small enough so that the short normal curve 

 is very little affected and shows low readings in front of the conductive layers. 



In soft formations of high porosity, the readings of the long normal 

 generally are little influenced by the mud and the invaded zone. Wherever 

 the bed thickness is great enough, a conductive bed is marked by a low apparent 

 resistivity, whereas a resistive bed gives rise to a high resistivity, sometimes 

 higher than the reading obtained with the short normal. Again for thick enough 

 beds, the comparison between the short and the long normal curves frequently 

 indicates whether the bed is invaded by mud filtrate: i.e., if it is permeable. 

 The beds, whose thickness is about equal to or lower than the spacing, are 

 very poorly shown by the long normal curve. 



The lateral curve opposite thick resistive beds is distorted, with a maximum 

 reading obtained in the lower portion of the bed. Sharp peaks are generally 

 obtained at the level of thin resistive beds, but the definition of these beds with 

 the lateral curve is often obscured by the presence of blind zones and spurious 

 peaks. Yet in deep invaded formations, such as hard-rock formations, only the 

 long lateral is able to give a reading not greatly affected by mud invasion. A 

 much more accurate definition of the boundaries of permeable beds, shales, 

 and hard streaks is obtained when the MicroLog is run together with the 

 conventional log. 



MicroLog 



A MicroLog is a resistivity log recorded from three electrodes arranged 

 in a vertical line, one inch apart, on a rubber pad. This pad is pressed against 



293 



