c.In interpretation, the unknowns will generally be 1, 2, 3 and 4. Granular materials will be high resistivecompared to fines such as silt and clay; crystalline materials (such as limestone or granite) will be highresistive compared to the granular materials.d.The quality of the formation water will greatly affect the measured resistively.In general, the resistively of a formation will vary in an inverse proportion to the total dissolved solids. For example, allother conditions remaining the same, if the total solid content increases, the formation resistively will decrease. Hence aclean sand filled with salty water may actually be extremely low resistive.e.Porosity of the formation also has an effect on the resistively. It is not as pronounced as the effect fromwater quality. In the logging of chemical precipitates, such as limestone, changes in porosity may enableyou to detect the water producing zones. Increased porosity will lower the formation resistively and hencein such material a low resistive zone (where no shale is present) is indicative of increased porosity. This isthen indicative of possible water production.f.The exact range of values for clean sand, gravel, or sandstone is something which you learn by experiencein your own particular area. In the midwest United States, clean sand and gravel generally exhibitresistively values in the range of from 350 to 1000 ohmft. The lower values apply to formations havingwater quality in the range of 300 to 400 ppm total solids and the upper values apply for formation watershaving 100 to 150 ppm total solids. The above remarks are, of course, very general and are included forguidance only.2-6.6 SelectingFormationContact. In "picking" the formation boundaries, the 0.25 ft curve should be used whereverpossible. The inflection point (the point midway between changes in curvature of the resistively curve) of the resistivelycurve is used to mark the contact between different formations.2-6.7 CorrelationbyElectricalLogs.a.A useful application of the electrical logs is in correlating formation thickness’ and depths from one well toanother. For example, two wells within a few feet of each other invariably will give identical electrical logs.When the wells are farther apart, the correlation will still be recognizable and the changes which do occur,as for example thickening or thinning of beds, are exactly the information needed to guide furtherexploration.b.Correlation is commonly possible to considerable distances in bedrock formation, in the order of thousandsof feet. Because of the variable nature of unconsolidated glacial and alluvial deposits, do not expect suchdistances except in special cases of a single, widespread type of deposit.2-6.8 TheEffectofMetalontheResistivelyLog.a.Because metal is such a good conductor, its presence in the zone of measurement, as for example airlines which have dropped to the bottom of the well, will cause a major decrease in the resistively and makethe log unusable, in so far as determining formation type. This effect, however, may be used to locatesuch steel in the well.2-11
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