The Minagish Formation in West Kuwait: A Comprehensive Sedimentological, Diagenetic and Reservoir Quality Characterisation

Abstract The Early Cretaceous Minagish Formation represents one of the best-performing carbonate reservoirs in West Kuwait. This study presents, for the first time, a complete sedimentological, diagenetic, and reservoir quality characterisation of all three Minagish Members (Lower, Middle, Upper) and constitutes a significant step forward in the understanding of the Early Cretaceous carbonate reservoirs in Kuwait. The study is based on the sedimentological description of 2,250ft of core that allowed the understanding of vertical and lateral facies variability across the three members. Core-to-log calibration helped the interpretation of uncored intervals and lateral well correlation. Petrographic and diagenetic analysis was conducted on 203 thin-sections to evaluate diagenetic overprint and pore system evolution. Reservoir quality evaluation was performed based on 2328 routine core analyses. The data were interrogated to unravel the sedimentological and diagenetic controls on reservoir performance. The Lower Minagish (LM) comprises dominantly low and moderate-energy mid-ramp deposits organised into shallowing-upward intermediate-order cycles capped by dolomitised horizons. Intercalation of organic-matter-rich deposits is noted towards the north, indicating the transition to deeper water conditions (outer ramp). The Middle Minagish (MM) describes an intermediate order cycle with a thick regressive trend described by a shallowing/coarsening upward from mud to grain-supported mid-ramp sediments to high-energy pack-grainstones and grainstones deposited in an inner ramp domain. These are variably laminated, reflecting the development of large shoals and intershoal areas. Abundant Lithocodium/Bacinella floatstones cap the sequence. The Upper Minagish (UM) is dominated by clay/organic-matter-prone wackestones to packstones reflecting deposition within an inner ramp setting subject to an influx of land-derived detrital material. This unit is likely to act as a seal or contribute to production through fractures. The best reservoir properties are hosted within the MM, where the high-energy grain-supported facies present the most effective pore system, thanks to a poorly cemented, well-connected interparticle macropore system. The LM and UM present much poorer reservoir quality due to their mud-prone and argillaceous character, which results in negligible porosity or a micropore-driven pore system (tortuous flow pathways). The mud-dominated nature of these sediments also triggers intense cementation, which further negatively affects the reservoir quality. Local presence of dolomite at the cycle top in the LM seems to subtly enhance reservoir potential. This integrated study highlights the very complex and heterogeneous sedimentological evolution of the Minagish Formation, which resulted in variable reservoir performance across the three study members. Whilst the grain-supported MM is well established as a high-performing reservoir, its internal heterogeneity, associated with different types of allochems, is captured here for the first time to enhance fine-scale predictability of vertical and lateral reservoir quality variations. Although LM and UM have conventionally been considered non-reservoirs, this study highlights the potential for production via micropores within the UM and via fractures and/or intercrystalline macropores in the LM.