Recognition and Prediction of Permeability in a Carbonate Ramp Environment, Abu Dhabi
By
Carlos Fonseca-Rivera1, Omar Al-Jeelani1, Martin Kraaijveld2
(1) Shell International E&P, B.V, Rijswijk, Netherlands (2) Shell UK Exploration and Production, Lowestoft, United Kingdom
This paper illustrates how field-wide permeability baffles may be predicted
by combining hi-resolution
FMI image analysis with cyclostratigraphy.
Image attributes of core-calibrated FMI logs have been used to identify
carbonate facies in a Lower Cretaceous carbonate ramp of Abu Dhabi. Recognized
facies in FMI logs include rudist, grainstone, Orbitolina, Bacinella and
bioturbated wackestones. The occurrence of facies is linked to third-order
eustatic variations in which low sea-level was associated with the development
of high permeable coarse-grained facies, and major flooding events with the
development of fine-grained low permeability facies. The gentle slope of this
carbonate ramp allowed the almost synchronous occurrence of major exposure and
flooding events fieldwide. In addition to the low order eustatic variations,
high frequency fluctuations in sea-level were responsible for the cyclic
occurrence of diagenetically-enhanced <1ft thick dense resistive streaks. These
streaks are believed to constitute vertical
flow barriers within the reservoirs
with some of them extending across several wells, and often undetected by non-microresistivity
logs.
The high and low permeability layers can be confidently detected at high
vertical
resolution
through image attributes of FMI logs. Spectral analysis of
the FMI attributes indicates that Milankovitch-band climate-forcing controlled
the occurrence of these thin resistive streaks, and therefore can be used to
predict their occurrence where FMI logs are not available.
This study has shown that the combination of image log attributes with gamma-ray, neutron and resistivity logs allows a reasonable prediction of permeability for the different facies in carbonate-ramps.