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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
AAPG Bulletin, V.
1Manuscript received November 18, 1996; revised manuscript
received June 29, 1998; final acceptance October 5, 1998.
2Comparative Sedimentology Laboratory, University of Miami,
RSMAS/MGG, 4600 Rickenbacker Causeway, Miami, Florida 33149; e-mail: [email protected].
Present address: Geologisches Institut, Swiss Federal Institute of Technology
ETH, Sonneggstrasse 5, CH-8092 Zürich, Switzerland.
3Comparative Sedimentology Laboratory, University of Miami,
RSMAS/MGG, 4600 Rickenbacker Causeway, Miami, Florida 33149.
ABSTRACT
Laboratory measurements on over 300 discrete carbonate samples reveal
that sonic velocity is a function not only of total porosity, but also
of the predominant pore type. In general, there is an inverse porosity-velocity
correlation, but significant deviations occur from this relationship for
certain pore types. Frame-forming pore types, such as moldic or intrafossil
porosity, result in significantly higher velocity values at equal total
porosities than do pore types that are not embedded in a rigid rock frame,
such as interparticle porosity or microporosity.
The results of the laboratory measurements can be applied to expand
interpretations of standard wireline-log data, as shown in this study on
two drill holes through Neogene carbonates from the Great Bahama Bank.
The velocity-deviation log is calculated by first converting porosity-log
data to a synthetic velocity log using a time-average equation. The difference
between the real sonic log and the synthetic sonic log can then be plotted
as a velocity-deviation log. Because deviations are the result of the variability
of velocity at a certain porosity, the deviation log reflects the different
rock-physical signatures of the different pore types. Positive velocity
deviations mark zones where velocity is higher than expected from the porosity
values, such as zones where frame-forming pore types dominate. Zero deviations
show intervals where the rock lacks a rigid frame, such as in carbonates
with high interparticle porosity or microporosity. Negative deviations
mark zones in which sonic log velocities are unusually low, caused, for
instance, by a cavernous bore-hole wall, fracturing, or possibly by a high
content of free gas. By tracing the velocity deviations continuously downhole,
one can identify diagenetic zones that are characterized by these different
pore types. In addition, this method can be used to observe permeability
trends because pore types influence the permeability of the rock.
The velocity-deviation log, which is calculated by combining the sonic
log with the neutron-porosity or density log, provides a tool to obtain
downhole information on the predominant pore type in carbonates. The log
can be used to trace the downhole distribution of diagenetic processes
and to estimate trends in permeability.
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