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The AAPG/Datapages Combined Publications Database

AAPG Special Volumes


Chapter from: M 61: Basin Compartments and Seals 
Edited by 
Peter J. Ortoleva

John N. Louie and Abu M. Asad

Methodology and Concepts

Published 1994 as part of Memoir 61
Copyright © 1994 The American Association of Petroleum Geologists 
All Rights Reserved


Chapter 9

Seismic Amplitude Versus Offset (AVO) Character of
Geopressured Transition Zones

John N. Louie
Abu M. Asad 
The University of Nevada
Reno, Nevada, U.S.A.



The presence of diagenetically cemented seals distinguishes between possible mechanisms for the maintenance of abnormal fluid pressures in sedimentary basins. The velocity gradients and Poisson's ratio variations surrounding a cemented seal affect the pre-stack amplitude versus offset behavior of seismic reflections from the seal. Acoustic synthetic seismograms based on well logs from sealed transitions demonstrate their unusual AVO character. A long-offset COCORP reflection line near Port Lavaca, Texas, shows similar effects from a seal at 6000 ft (1800 m) depth. Local conductivity logs and regional drilling mud weight compilations establish the presence of this pressure transition zone. It is associated with a strong, low-frequency reflector near its base and displays linear AVO trends markedly stronger than from deeper reflections within the overpressured compartment. The fact that a seal exhibits such prominent physical property characteristics suggests that AVO analysis techniques may locate other seals in basins worldwide, where proper conditions for diagenetic sealing rather than shaliness and subsidence may be needed to maintain geopressures.

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