# Effect of overpressure

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Series Geophysical References Series Problems in Exploration Seismology and their Solutions Lloyd P. Geldart and Robert E. Sheriff 5 141 - 180 http://dx.doi.org/10.1190/1.9781560801733 ISBN 9781560801153 SEG Online Store

## Problem 5.9a

Figure 5.6a shows velocity versus depth for normally pressured shales. How do the Figure 5.9a velocities above and below the top of the abnormal pressure zone compare with the sand and shale velocity-depth curves of Figure 5.6a? What depth corresponds to normal pressure for the top overpressure? What porosity would you expect for the overpressured shale?

### Solution

The transit times above and below the “top of the overpressure zone” are roughly ${\displaystyle (125\ \mu s/{\hbox{ft}})}$ and ${\displaystyle (175\ \mu s/{\hbox{ft}})}$, respectively, that is, velocities of 8.0 and 5.7 kft/s (2.4 and 1.7 km/s). According to Figure 5.6a, the shale velocity at 6000 ft (2.1 km) should be higher (about 8.8 kft/s); the shale is probably somewhat undercompacted. The 5.7 kft/s velocity of the overpressured shale corresponds to a depth of about 1000 ft (300 m) and the porosity of the overpressured shale is probably close to 50%.

Figure 5.9a.  Effect of overpressure on sonic and resistivity logs for an offshore Gulf of Mexico well (from MacGregor, 1965).

## Problem 5.9b

Plot the velocities for 100% water saturated sands from Figure 5.8a(i) on Figure 5.6a. How do they compare?

### Solution

The values for 100% water saturated sands from Figure 5.8a(i) are listed below and plotted as triangles on Figure 5.6a. They lie slightly below the Gulf Coast sand curve.

 Depth 0.61 km 1.83 3.05 Velocity 1.9 km/s 2.5 3

The data from Figure 5.9a are plotted as squares on Figure 5.6a.