Page 73 - Geology of Carbonate Reservoirs

54 CARBONATE RESERVOIR ROCK PROPERTIES

If a 10 - m thick gas reservoir in microporous limestone occurs in a deep (several
thousand meters) subsurface limestone interval several times as thick as and similar
in lithology to the porous zone, the reservoir will be invisible to the seismograph.
The thickness of the porous zone in such a case is below the limit of separability or
one - quarter the seismic wavelength, as illustrated by Brown (1999). A rule of thumb
is that beds of roughly 15 − 20 - m thickness will, on average, be within the required
one - quarter wavelength at moderate subsurface depths. The earth acts as a fi lter
and progressively eliminates high - frequency acoustic waves with increasing depth
in the subsurface. Small - scale variations in reservoir properties require small wave-
lengths (higher frequencies) to be “ seen. ” Seismic measurements of reservoir char-

acteristics depend on quality of the reflection data. In other words, for optimal

reflection the depth of investigation must be shallow, the signal must be strong and

not corrupted by noise, and the acoustic impedance must be sufficient to cause good

reflections. The seismograph can be a powerful tool to help identify reservoirs and,
in some cases, flow units within reservoirs. Anselmetti and Eberli (1997) studied

seismic compressional and shear wave velocities ( V p and V s ) in 295 minicores and
found, much as Wang (1997) did in his laboratory study, that different seismic veloci-
ties in rocks of equal porosity were the result of different pore types. It is possible,
under ideal conditions, to estimate carbonate rock and reservoir properties based
on their seismic reflection characteristics. But as Lorenz et al. (1997) emphasize,

there are caveats in making subsurface interpretations where the scale of the seismic
measurement is larger than the scale of the individual reservoir interval.

SUGGESTIONS FOR FURTHER READING

G. B. Asquith and D. Krygowski ’ s (2004) Basic Well Log Analysis , 2nd edition, is a
standard reference, especially for using nomograms to obtain values for porosity,
saturation, and lithology from wireline log data. This edition includes new material
on magnetic resonance imaging and borehole imaging logs. A thorough discussion of
logging methods, how the logging tools work, and how to interpret log data is given
in M. Rider ’ s (1996) Geological Interpretation of Well Logs , 2nd edition. Additional
references on seismology include W. M. Telford, L. P. Geldart, and R. E. Sheriff (1991) ,

Applied Geophysics , 2nd edition; J. Milsom (2003) , Field Geophysics , 3rd edition; and
R. E. Sheriff (2002) , Encyclopedic Dictionary of Exploration Geophysics , 4th edition.
A widely cited book on 3D seismology is by A. R. Brown (2004) , Interpretation of
Three - Dimensional Seismic Data , 6th edition. A good compilation of research papers
specifically on carbonate seismology can be found in I. Palaz and K. J. Marfurt (1997) ,

Carbonate Seismology . For those wishing to see color images of carbonate particles,
pore types, and effects of diagenesis on carbonate rocks, A Color Guide to the Petrog-
raphy of Carbonate Rocks: Grains, Textures, Porosity, Diagenesis , by P. A. Scholle and
D. Ulmer - Scholle (2004) , is an excellent starting point.

REVIEW QUESTIONS

2.1. What are fundamental and derived rock properties?
2.2. What is the difference between texture and fabric?