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Abstract

AAPG Memoir 76, Chapter 18: Previous HitVelocityNext Hit Estimation for Pore-Pressure Prediction , by David W. Bell, Pages 177 - 215
from:
AAPG Memoir 76: Pressure Regimes in Sedimentary Basins and Their Prediction, Edited by Alan Huffman and Glenn Bowers
Copyright © 2001 by The American Association of Petroleum Geologists. All rights reserved.

Chapter 18
Previous HitVelocityNext Hit Estimation for Pore-Pressure Prediction

David W. Bell
Conoco Inc.
Ponca City, Oklahoma


ABSTRACT

The speed of propagation of compressional-wave energy in the subsurface, known simply as “formation Previous HitvelocityNext Hit,” is strongly influenced by compaction, particularly in young clastic basins. Because pore pressures affect compaction, changes in Previous HitvelocityNext Hit can be calibrated to changes in pore pressure. Velocities derived from surface seismic data provide indirect pressure measurements at undrilled locations. The accuracy depends on the validity of the relationship between pressure and Previous HitvelocityNext Hit, the quality of the Previous HitvelocityNext Hit measurements at enough points to perform the calibration and prediction, and the reliability of average velocities to correctly convert from seismic time to Previous HitdepthNext Hit.

A key step is construction of a Previous HitvelocityNext Hit profile with Previous HitdepthNext Hit that simultaneously defines both the compaction characteristics and a valid time-Previous HitdepthNext Hit curve. A linear fit to the logarithm of the sonic transit time with Previous HitdepthNext Hit is commonly assumed to represent the normal compaction trend. Such a Previous HitvelocityNext Hit-Previous HitdepthNext Hit trend, however, does not produce a time-Previous HitdepthNext Hit relationship that accurately converts seismic measurements in time to Previous HitdepthNext Hit. A linear fit of Previous HitvelocityNext Hit with time provides a consistent fit to both time-Previous HitdepthNext Hit and Previous HitvelocityNext Hit-Previous HitdepthNext Hit data and is a better empirical representation of the normal compaction trend. The linear Previous HitvelocityNext Hit-time model can be used to smooth through inaccuracies in seismic stacking-Previous HitvelocityNext Hit picks where applied to geologically consistent units.

This chapter illustrates relationships between Previous HitvelocityNext Hit and the geologic setting and establishes an empirical model for the normal compaction trend. It then reviews various assumptions and techniques for converting seismic stacking velocities into representative formation velocities. It concludes with a step-by-step recommendation for estimation and calibration of Previous HitvelocityTop from seismic data.

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