338 



Subsurface Geologic Methods 



where an abrupt break in the density curve marks the unconformity be- 

 tween the Miocene and Eocene. 



Thrust faulting may, in some instances, be detected through careful 

 shale-density studies. This is particularly true in cases where displace- 

 ment due to thrusting involves several thousand feet of section. 



Weathered zones, particularly at major unconformities, can usually 



AVERAGE SHALE DENSITIES BY lOOOFT. INTERVALS, 

 FOR laS SAMPLES FROM 5 WELLS IN COLOMBIA 

 LA PUERTA No.l, U-3^ CIMITERRA No. I, 

 EAST INFANTAS No.5, WELL 714. 



NUMBER OF SAMPLES, INDICATED FOR EACH AVERAGE. 

 SAMPLES DESCRIBED AS 'SANDY SHALE"OR 'LIMEY SHALE" 

 WERE NOT INCLUDED IN THE AVERAGES. 

 DATA FROM C.H. ACHESON'S MEMORANDA 



9.000 10.000 



Figure 142. Average shale densities from Colombian wells. Note that densities are 

 higher than averages for all samples. (After Skeels.) 



