604 Subsurface Geologic Methods 



Distribution of Residual Magnetic Minerals in a Core 



Under ideal conditions we assume the ferromagnetic minerals to be 

 regularly distributed throughout the core and thus to give rise to a uniform 

 magnetic field. It is possible, however, that isolated crystals having resi- 

 dual magnetism may be present near the surface of the core. Because of 

 this advantageous position they may exert an abnormal effect on the mag- 

 netic field of the entire core. The operator may be able to detect this 

 abnormality of position, however, and correct it empirically. 



Rock Movements 



A history of the movements undergone by the sedimentary rocks 

 should be taken into consideration in magnetic orientation. Rock move- 

 ments that most concern magnetic orientation of cores are (1) tilting of 

 rock after deposition, (2) stress and strain effects, and (3) high-tempera- 

 ture effects. 



Tilting — Tilting of sedimentary rocks is the most obvious and wide- 

 spread geologic factor of error under the general heading of "Rock Move- 

 ments." A certain amount of tilting is associated with every oil field, 

 ranging from a maximum in highly folded and faulted structures to a 

 minimum in types of stratigraphic traps. In all cases we must remember 

 that any directional magnetism that was imparted to the matrix of the 

 sediment was accomplished at the time of deposition, unless the polarity 

 was later altered by high temperatures. Subsequent movements rotate the 

 rock and the residual magnetic field of the rock as a unit. 



Stress and Strain Effects — We may assume the effect of stress and 

 strain on ferromagnetic minerals to be somewhat similar to that on crys- 

 tals of ferromagnetic metals and alloys. These materials display the prop- 

 erty of magnetostriction, i.e., they either contract or lengthen a small 

 amount when magnetized. The effects of strain on magnetism and mag- 

 netism on strain are interdependent. In other words, the magnetism of 

 some materials (those that lengthen when magnetized) is increased by 

 tension; that of others (those that contract when magnetized) is de- 

 creased by tensional strain. Likewise, in weak magnetic fields some mate- 

 rials have positive magnetostriction (lengthen) and in strong magnetic 

 fields negative magnetostriction (contract) . 



In highly folded rocks, where large external forces are active, the 

 .strength and possibly the direction of magnetism may be altered in a 

 fashion to comply with strain-ellipsoid relationships. However, we believe 

 this consideration to be of more theoretical and less practical importance 

 as measured by core-orientation standards. The normal association of 

 high temperature with high pressure may make it difl&cult to single out 

 the effects of one in the presence of the other. 



Temperature — High temperature destroys residual magnetism. The 

 temperature above which residual magnetism ceases to exist is called the 



