Effect of overpressure

Problems in Exploration Seismology and their Solutions

Series	Geophysical References Series
Title	Problems in Exploration Seismology and their Solutions
Author	Lloyd P. Geldart and Robert E. Sheriff
Chapter	5
Pages	141 - 180
DOI	http://dx.doi.org/10.1190/1.9781560801733
ISBN	ISBN 9781560801153
Store	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 $(125\ \mu s/{\hbox{ft}})$ and $(175\ \mu s/{\hbox{ft}})$, 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	3.0

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

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Table of Contents (book)

Also in this chapter

• Maximum porosity versus depth
• Relation between lithology and seismic velocities
• Porosities, velocities, and densities of rocks
• Velocities in limestone and sandstone
• Dependence of velocity-depth curves on geology
• Effect of burial history on velocity
• Determining lithology from well-velocity surveys
• Reflectivity versus water saturation
• Effect of overpressure
• Effects of weathered layer (LVL) and permafrost
• Horizontal component of head waves
• Stacking velocity versus rms and average velocities
• Quick-look velocity analysis and effects of errors
• Well-velocity survey
• Interval velocities
• Finding velocity
• Effect of timing errors on stacking velocity, depth, and dip
• Estimating lithology from stacking velocity
• Velocity versus depth from sonobuoy data
• Influence of direction on velocity analyses
• Effect of time picks, NMO stretch, and datum choice on stacking velocity

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