Page 225 - Geology of Carbonate Reservoirs

206 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

The seismograph records shock waves reflected from acoustic interfaces in the

subsurface. It does not record texture, grain type, sedimentary structures, or taxo-
nomic diversity and it cannot distinguish directly between depositional, diagenetic,
or fracture porosity. Those distinctions have to be inferred by seismic interpreters
who, with modern seismic data acquisition and processing techniques, can use refl ec-
tion amplitude, frequency, phase, polarity, spatial extent, and shear wave character-
istics to help identify seismic signatures of reservoirs, particularly gas reservoirs,

because gas - filled pores react to seismic pulses much differently than do oil - fi lled

pores (Brown, 1999 ). Seismic reflectivity depends largely on impedance contrast
between the target rock layers and those that enclose it. If a 10 - m thick gas reservoir
in microporous limestone occurs in a deeply buried carbonate sequence several
times as thick as the reservoir zone, that reservoir will be below the limits of detec-
tion by the seismograph. There will not be enough impedance contrast and the
thickness of the porous zone will be below the limit of separability or one quarter
the seismic wavelength, as illustrated by Brown ( 1999 ). If the limit of separability
is not exceeded and if the impedance contrast between reservoir and enclosing
nonreservoir rocks is big enough, 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 velocities in rocks of equal porosity were the result of different
pore types. It is possible, under the right conditions, to estimate carbonate rock and
reservoir properties based on their seismic refl ection 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.

8.2 DATA REQUIREMENTS

The amount of uncertainty involved with finding and developing carbonate reser-
voirs depends on how much is already known from previous work in a prospect
area. It is relatively easy to diagnose that an oolitic grainstone is a beach – dune
deposit on a ramp if it is already known that the depositional succession was depos-
ited on a ramp and that the drilling location is in a depositional dip position that
should place it near the ancient strandline, or “ pinchout edge ” for that formation.
It is not easy to determine the depositional model for a rank wildcat in an untested
basin where the kind of platform is poorly understood, the depositional dip position
in the target formation has not been established, and only a few old, 2D seismic
records are available. To make it worse, if cores and full complements of borehole
logs are not taken in such frontier exploration ventures, little will be learned from
the failures. Without the necessary information about the geology of the fi rst failure,
more failures will follow. What kinds of information are necessary to explore for
and develop carbonate reservoirs while minimizing risk and uncertainty? The infor-
mation exists in a hierarchy of scales that extend from regional (platform or basin
scale), to field scale (one or more reservoirs), to reservoir scale (one or more depo-

sitional or diagenetic facies or fracture zones), to flow unit scale (subdivisions of