Petrography, sedimentology, palynomorphs and diagenesis of the Paleozoic Khuff and Unayzah formations in Kuwait

Integrated geological investigations using rock cores and well logs were performed on the Paleozoic Khuff and Unayzah formations to study their petrography, sedimentology and diagenesis. The formations occur at depths >15,000 ft, while the Unayzah was not reached in some wells up to 20,000 ft. Their depositional basins seem to be affected by the Kuwait Arch, where the Khuff Formation is fully penetrated only in the wells located over this Arch. The Khuff Formation shows complex lithological facies composed mainly of massive hard, fractured dolostone with frequent anhydrite veins and nodules. The Dolomite concentration ranges from 90 to 100% with varying percentage of anhydrite and low quartz percent (2.5%). Few minerals such as quartz, montmorillonite and pyrite are found at some depths. The Basal Khuff Clastics (BKC) is represented by a sequence of interbedded sand, muddy sand and carbonaceous shale with advanced diagenetic changes, deposited as calcareous grainstone within shallow marine environment (neritic zone) and has been diagenetically altered through several stages of dolomitization. Quartz concentration ranges from 5.3% to 100%, increasing with depth with relatively low percent of muscovite, clay mineral, illite and montmorillonite, microcline, pyrite and hematite. By contrast, the Unayzah Formation shows a fluvial siliciclastic sequence of conglomerate, sandstone, siltstone, mudstone and shale. Four sedimentation cycles of different thickness are recognized within the formation, each cycle starts with fining upward sandstone that grades to mudstone with shale intercalations and ends with red paleosoil sedi-ments. The sandstone is a mixture of quartz, muscovite, and albite minerals. The polynomorphs study suggests that the formation was deposited in alluvial, fluvial, braided streams and a shallow marine environment. Signs of local incipient metamorphism are detected in the Unayzah clastics south of the Burgan Field may interpreted as a local phenomenon due to the lack of matching with the present-day normal thermal regime.