the value of the true resistivity from the apparent resistivities read with the 

 conventional devices. 



The curves show the departure of apparent resistivity from true resistivity 

 as a function of the various related parameters. 



K t = the true resistivity of the formation 



K m = the resistivity of the mud in the drill hole opposite the formation 

 d = the diameter of the drill hole 



AM or AO = the spacing of the device used to make the measurement 

 e = the thickness of the bed (in a bed of finite thickness) 

 R s — the resistivity of the adjacent beds (in a bed of finite thickness) 

 Rj and D{ = parameters determining the invaded zone. (In the compu- 

 tation of the curves, the invaded zone is supposed to be a 

 homogeneous medium of resistivity Rj, separated from the 

 uncontaminated zone by a cylindrical surface of diameter 

 Di, co-axial with the borehole) 



In the presentation of the charts, the effective number of variables is 

 reduced by using the diameter (d) of the drill hole as the unit of length and 

 the resistivity of the mud (R™) as the unit of resistivity. The list of variables 

 is thus shortened to Ra/R^, R t /Rm, R-i/Rm? Rs/Rm> AM/d, or AO/d, Di/d, and 

 e/d. The curves are grouped on the charts according to whether they correspond 

 to normal or to lateral devices, according to whether they correspond 

 to beds of infinite thickness or to beds of various finite thicknesses, and accord- 

 ing to the conditions of invasion. 



The resistivity departure curves are not of great help for the estimation of 

 R 4 in thin beds (thickness less than about 15 feet, 20 feet when bed resistivity 

 is moderate) because the effect of the adjacent formations on the readings 

 made opposite the beds is so great that the curves have practically no resolution. 

 The curves are useful for thick beds, provided the value to be taken for R^ is 

 approximately known (this value may be equal to K x0 , as given by the Micro- 

 Log or MicroLaterolog) and that some reasonable hypothesis can be made on 

 the depth of invasion. In general the departure curves are valuable to assess 

 the respective influence on the apparent resistivities of the factors involved and 

 to determine what response should be expected from the various measuring 

 devices in each given condition. 



Application of Conventional Electric Log 



In sand-shale series, geological strata generally form long sequences of thin 

 sand and shale layers. Usually a sand line and a shale line can be traced on 

 the SP logs showing that many sands are practically clean of shaly material 

 and that many shales do not contain an appreciable amount of sand. 



292 



