Velocity is a function of the elastic constants of the formation and as such 

 represents a new logging parameter. Velocities increase as the degree of com- 

 paction and the geologic age increase. Shales may be identified on the log by 

 their relatively low velocities. Higher velocities are indicative of sandstones, 

 limestones and dolomites. The resulting logs are very readily correlatable 

 between wells. 



In addition to its use for correlation and for qualitative analysis of sedi- 

 ments, the CVL log also offers measurements of the over-all average velocity 

 through the logged section. The total travel time through the section is computed 

 by the integrating device in the instrument. This travel time is portrayed by the 

 integrated curve, and when used in conjunction with two or more recorded travel 

 times to a well geophone, yields accurate average velocity information to the 

 various horizons logged. 



EXAMPLES AND Figure 21-5 shows a continuous velocity log 



APPLICATIONS OF of a well in Caldwell County, Texas, along 



VELOCITY LOGS with the geologic section, and demonstrates 



the relationship between lithology and velocity. 

 Velocity Log Relates The Lower Cretaceous section shown includes 



Lithology and Velocity the Hensel, Cow Creek, and Sligo formations 



and penetrates the Hosston. The more prom- 

 inent shale and limestone breaks are indicated on the geologic log on the left. 

 The presence of the thin shale breaks at 3580 and 4325 feet are effectively 

 demonstrated by the interval velocity curve on the right. The over-all average 

 velocity is obtained from the integrated travel time curve shown on the log as 

 a fairly straight line. This curve corresponds with the familiar time-depth 

 curve obtained from travel times to a well geophone. 



Velocity Logs Compared With Electrical Logs 



Comparison of electrical and acoustic characteristics is shown in Figure 21-6. 

 These are logs from two wells in Gonzales and Caldwell counties, Texas, separ- 

 ated by a distance of more than 50 miles. The self-potential, resistivity, and 

 interval velocity curves are displayed for each well, and the identification of 

 formations indicated. The character correlation of the Eagle Ford section is 

 particularly noticeable, despite reduction in thickness in the Caldwell County 

 well. Also of interest is the velocity relationship between the Georgetown and 

 Edwards limestones as compared with their electrical curves. The increased 

 resistivity shown at the top of the Georgetown and again at the top of the) 

 Edwards at the Caldwell County well (left) is not maintained at the Gonzales 

 County well (right). The velocity log, however, reveals identical correlation 



413 



