The bioturbated chalky limestones of the Khasib Formation (Upper Turonian - Lower Coniacian) furnish<br>an extensive reservoir rock of several oil fields in central Iraq including: Tikrit, Balad, Samarah, and<br>East Baghdad oil fields. Investigation of these rocks by means of microfacies analysis and electrofacies<br>correlation as well as sequence stratigraphic analysis indicate that it had being deposited in a ramp<br>setting sloping east- southeastwards forming part of the Arabian passive margin.<br>The inner ramp is characterized by a bioclastic packstone of a carbonate bank with green shale<br>intercalations of peri-bank sediments. The middle ramp which dominate the Khasib Formation section<br>is consist of white- beige, porous, bioturbated, chalky and dolomitic bioclastic wackestone. Bioclasts<br>includes; shelf faunal debris with variable mixture of benthic and planktonic forams. Intensive<br>Thalasionodes and Palaeophycus bioturbations significantly contributed to the high porosity of this part.<br>The outer ramp is consists of intercalations of boiturbated bioclastic chalky limestone and basinal<br>argillaceous limestone. The latter is characterized by the occurrence of planctonic forams, calcispheres,<br>dwarf rotalids and sponge spicules.<br>The Khasib section represents a third order cycle with lower sequence boundary of type one separating<br>Khasib sequence from the underlaying LST of the Kifl Formation. The TST is consist of thin and basinal<br>facies. The HST is the thickest and represented by the bioturbated chalky limestone of the middle ramp<br>facies. The maximum flooding surface is idicated by a thin horizon dominated by intensive Paleophycus<br>bioturbation within the middle ramp facies. The boundary with the overlaying Tanuma Formation is of<br>type two and represented by the transition to the LST of the next cycle.


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