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.