Diagenetic imprints and their impact on reservoir characterization of the Jurassic Middle Marrat Member, northern Kuwait

Abstract

The Lower Jurassic Marrat Formation in Kuwait is divided into Lower, Middle and Upper members. The Middle Marrat section is the most prolific Jurassic gas and oil producer in northern Kuwait. It was deposited in an inner to middle ramp setting and consists of grainstones, packstones, wackestones, with mudstones, algal packstone, anhydrite and crystalline dolomite in minor proportions. The dominant allochems are oncoids, ooids, peloids, intraclasts, echinoiderms, molluscs, gastropods and forams. Anhydrite mostly occurs as nodules and show chicken-wire texture with rare bedded anhydrite. Reservoir quality is highly variable in the Middle Marrat section and is mostly independent of depositional facies. Both primary and secondary matrix porosity, as well as fracture porosity, are present, with rare styloporosity. Primary inter-particle pore spaces are found in grainstones, and the porosity has been mostly reduced by isopachous and blocky calcite cements. Scattered moldic porosity is present. Secondary inter-crystalline porosity is formed by replacement of calcite by dolomite. Good preserved inter-crystalline porosity is seen in completely dolomitized sequences having sucrosic texture. Multiple diagenetic events from micritization to fracturing have been observed. Dolomitization is the dominant control on the diagenetic modification and both mimetic and non-mimetic dolomites are present. Dolomitization was caused by either evaporative reflux or hydrothermal processes. Anhydrite occurs as early cement associated with sabkha evaporation as well as late burial cement. Late stage diagenetic events are represented by poikilotopic anhydrite, pyrite, saddle dolomite, calcite cement (including rare dedolomite) and fractures. Crystalline dolomite is formed by the replacement of calcite and represents diagenetic rather than depositional facies. The better understanding of the diagenetic pattern of the Middle Marrat section has allowed the selection of favourable location for delineation drilling.