Depositional and burial history controls on the diagenetic evolution of interbedded sandstone and shale: application to the Ordovician Khabour Formation, Iraq
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Petrographic (optical, scanning, and backscattered electron microscopy), mineralogical (X-ray diffraction) compositional analysis of interbedded sandstones and shales from the Ordovician-age Khabour Formation of western Iraq were investigated in order to determine the diagenetic evolution of these units. The Khabour sandstones are generally quartzarenites with subordinate sublitharenites and subfeldsarenites which were deposited in a range of shelf environments (offshore shelf through tidal-storm regressive middle shelf to near-shore inner shelf). Several diagenetic events have affected the Khabour sandstones including compaction, cementation, replacement, dissolution and alteration. Early diagenetic events were closely related to the composition of depositional water (especially in terms of oxygen and sulphate content), Fe and organic content of the sediments, rate of sedimentation, and proximity to the shoreline. Shales interbedded with the sandstones contain evidence of authigenic illite, chlorite, kaolinite and mixed-layer illite-smectite. The later diagenetic events within the Khabour Formation were related to the burial history of the sandstone and shale succession as it was deposited on the slowly-subsiding shelf. Typical features include quartz overgrowths, the development of authigenic kaolinite, chlorite and illite, albitization, as well as Fe-dolomite and titanium and ferruginous cementation that resulted in decreased permeability by reducing pore-throat openings in the studied sandstones.