Dependence of velocity-depth curves on geology

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Problem 5.5a

Why do the velocity-depth curves for various areas shown in Figures 5.5a and 5.5b depart from each other? Incorporate your knowledge of the geology of the various areas in your answer.

Solution

Velocities are increased not only by compaction with depth of burial but also by cementation and other factors attendant upon age. They are also affected by lithology and other factors. The U.S. Gulf Coast and offshore Venezuela sediments are predominantly young siliciclastics that have never been buried deeper than they are now. Hence their velocities generally relate to the maximum porosities such as shown in Figure 5.1a. Similar reasoning applies to the shallower portions of the offshore U.S. East Coast and the Gulf of Alaska curves of Figure 5.5b. The high-velocity values in the upper part of the Gulf of Alaska curve probably indicate limestone or volcanics. The Texas Gulf Coast-1 curve penetrated Cretaceous rocks containing limestone at fairly shallow depths whereas the Gulf Coast-2 well did not encounter this section until a depth of 4.25 km. The Illinois Basin and Permian Basin wells contain much older, higher-velocity rocks including limestone.

Figure 5.5a.  Velocity-depth, U.S. Gulf Coast.

Problem 5.5b

Plot the shale and limestone values from Figure 5.5c for depths of 1000 and 2000 m on the velocity-depth Figure 5.5a. How do they compare?

Figure 5.5b.  Velocity-depth, selected wells.

Solution

The values read from Figure 5.5c are:

shale at 1000 m 2.2 km/s
shale at 2000 m 2.8
limestone at 1000 4.2
limestone at 2000 5.4
Figure 5.5c.  Velocity/depth relations (from Jankosky, 1970).

These points are plotted as triangles on Figure 5.5a for regions where the sections are sand-shale. The shale values fit nicely, but the limestone values are much too large, as one would expect. The limestone values are also plotted on Figure 5.5b where they fit in nicely with data for regions that are mostly carbonate.

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