Stylolite in Upper Cretaceous Carbonate Reservoirs from Northwestern Iraq

Stylolites are commonly observed in the carbonate reservoirs in various oilfield of Iraq including those of upper Cretaceous successions from northwestern Iraq, where they are characterized by stylolite-rich zones in the Cenomanian-early Turonian Gir Bir Formation and to a lesser extent in the Turonian-Santonian Wajna and early Campanian Mushorah formations respectively. The observed stylolites are either large to be identified in the core samples or smaller ones that are well observed in the thin sections and are characterized by variations in amplitude, morphology and accumulated insoluble residues. The recorded stylolites are classified as hummocky, irregular, low and high-amplitudes peaks, and irregular anastomosing stylolites. Stylolites affect the porosity permeability and thickness reduction compaction as the main chemical compaction (pressure solution) that reduce porosity. Whereas, in other places, the stylolites act as seals and stop the upward movement of hydrocarbons. This is also seen for mineralization processes such as silicification that ended near the stylolite surfaces

Depositional and burial history controls on the diagenetic evolution of interbedded sandstone and shale: application to the Ordovician Khabour Formation, Iraq

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