Experience

Reservoir quality analyses and prediction - Zubair Formation, Kuwait

Study aims: to evaluate the controls on reservoir quality, and integrate these results with the sedimentological framework in order to better understand reservoir quality distribution within the Zubair Formation.

Database: 4 cored wells from Raudhatain Field, 274 thin-sections, 50 XRD, 26 SEMs, MICP data from 2 wells.

Approach: the main textural, compositional, diagenetic and pore system characteristics were categorised and qualitatively tabulated for all thin-sections, integrated with CCA data, and form the basis for rock-type assignment. These were calibrated with quantitative textural and modal data from a subset of 100 samples and available MICP data, prior to upscaling results within the context of the sedimentological framework.

Integrated results: the Zubair Formation comprises highly mature deltaic clastic deposits with variable, heterogeneously distributed argillaceous matter. The sandstones dominantly range from very fine to medium-grained and are only weakly to moderately overprinted by authigenic cements. Reservoir quality within the Zubair Formation sandstones is essentially controlled by the primary depositional detrital clay content (Plot A), with additional control by grain size (and minor quartz cementation within the cleanest deposits, Plot B). However, as a result of facies, bed and plug-scale heterogeneity in detrital clay content, the linkage between reservoir quality and the sedimentological descriptors is poor, especially within the highly heterogeneous non-channelised deposits. The underlying variations in pore system characteristics have been used to define “rock types”, which show better correlation with reservoir quality trends than the coredescriptive lithotype scheme and can potentially be integrated with MICP data. These rock types show distinct ranges of occurrence within the individual facies associations (C) and hence allow reasonable predictability of likely reservoir character within the sedimentological framework (D).