velocity logs of two wells in central Oklahoma and two wells in southern Okla- 

 homa revealing good correlative quality. The wells in Figure 21-11 are about 

 30 miles apart and the wells in Figure 21-12 are y 2 mile apart. The deep pre- 

 Pennsylvanian correlations are distinct in both illustrations. The character of 

 the Pennsylvanian section shown in Figure 21-12 is considered typical for 

 Pennsylvanian logs. 



Figure 21-13. Velocity log correlations over long distances. 



As demonstrated in Figure 21-5, the log is considered unique in that it 

 portrays the relationships between velocity and lithology, and the influence 

 of compaction and geologic age. The log provides a direct measurement of a 

 physical property of the formation. Therefore, it is possible in many instances 

 to identify the log geographically by simply associating the observed velocities 

 to lithology and reasonable geologic age. For example, a Gulf Coast log is 

 easily identified by the thick sections of relatively low-velocity material and 

 Oklahoma logs are easily identified by the distinct Pennsylvanian and pre- 

 Pennsylvanian velocity contrasts. 



The excellent quality of velocity log correlations over long distances is 

 revealed in Figure 21-13. The top of the Niabrara is shown to virtually write its 

 name on eight logs of wells located over distances in excess of 600 miles across 

 North Dakota, South Dakota, Wyoming, Nebraska, and Colorado. 



419 



