The Late Albian Mauddud Formation of North Kuwait is composed of inner ramp carbonates and deltaic clastics. It hosts prolific hydrocarbon reserves in Kuwait and across the Arabian Gulf region where the reservoirs are typically the carbonate deposits. Accurately predicting the reservoir properties in the Mauddud Formation is challenging due to the non-unique wireline signatures, which result from the inherent diagenetic heterogeneity and complexity. 385ft of continuous FMI image log data, collected from a single well covering the entire formation in a north Kuwait field, has been analysed to assess its potential to characterise and extrapolate the sedimentological, diagenetic and reservoir characteristics into uncored wells. Nine distinct image facies have been defined, each of which display a specific image fabric. The image facies have been calibrated with core, thin section petrographic observations and core plug data to aid reservoir quality predictability.
The study shows that gross sedimentological and diagenetic characteristics observed in the core, including laminations, patchily distributed cements and bioturbation are identifiable in the image logs. The image facies display mottled, laminated or massive fabrics. The argillaceous, fine-grained deltaic clastics, which are non-reservoir, correspond to the finely laminated image facies, however, these deposits are easily identified in uncored intervals by their distinct wireline log signature. The mottled image facies, which is associated with patchy calcite and dolomite cemented carbonates, are subdivided into six categories primarily based on the mottling size. This work establishes a systematic relationship between the mottling size and the grain to micrite matrix ratio of the deposits, and hence, the depositional setting. The finest mottles, corresponding to the smallest cement patches, are typically associated with micrite-supported lithofacies representing deposition in a low-energy inner ramp environment, while the coarsest mottles are principally observed in the grain-supported carbonate lithofacies, which are prevalent in higher energy inner ramp environments. This relationship, which possibly relates to differences in the bioturbation and/or diagenetic overprint of the deposits, is used as a proxy to interpret the depositional energy in the Mauddud Formation. Cross-laminated and massive image facies are also locally identified and are predominantly associated with high-energy shoal complex carbonates. The image facies, in part, also predict the distribution of some of the best porosity and permeability in this formation, notably in the finer mottled image facies where reservoir quality is good to moderate (HeΦ 12.5-25.8% and Kair 1.04-22.4mD). In the coarser mottled image facies, reservoir quality is heterogeneous, but can be comparably good.
Using this approach, the gross rock and reservoir properties of the Mauddud Formation can be partly characterised using FMI data alone. The scheme will be refined using further core-calibrated FMI datasets from additional wells to ultimately aid the prediction of reservoir quality at field-scale.