Sedimentology and reservoir characteristics of the\ud oligocene-early miocene carbonates (Kirkuk group)\ud of southern Kurdistan
Kirkuk Group Formations (in addition to Avanah and Jaddala Formations) of\ud southern Kurdistan were studied in order to determine biostratigraphy,\ud chronostratigraphy and sequence stratigraphic relationships, in addition to major\ud sediment producing environments and type of platform configuration. As well as to\ud determine the paragenetic sequences with special attention to micrite diagenesis\ud and its effect on microporosity.\ud Five biozones were identified in the study area in which two of them from Middle-\ud Late Eocene: Alveolina biozone (AL) and Discocyclina biozone (DI) with three\ud biozones from the Oligocene-Early Miocene of Kirkuk Group: Nummulites fichteli\ud biozone (NF); Praerhapydionina delicata biozone (PD) and Austrotrillina howchini\ud biozone (AH).\ud Twenty two microfacies were identified and interpreted as having been deposited in\ud a ramp setting based on lateral variations of the microfacies; gradual deepening with\ud no evidence of slope break or effective barrier. A depositional model has been\ud generated from the overall palaeoenvironmental interpretations of the microfacies in\ud which the analysed microfacies indicates palaeoenvironments ranging from\ud terrestrial to open marine settings; nine major depositional environmental zones\ud have been identified and correlated with the standard Cenozoic ramp model of\ud Buxton and Pedley (1989). These zones distributed across the ramp setting dipping\ud southwest, in which zone 1 is terrestrial deposit; zone 2, 3, 4 and 5 are belonging to\ud inner ramp; zone 6, 7 and 8 are belong to middle ramp and zone 9 is belong to outer\ud ramp and basinal settings.\ud A paragenetic sequence has been derived recording eleven diagenetic processes\ud affecting the Kirkuk Group which are subdivided into an eogenetic, mesogenetic and\ud telogenetic stages. Furthermore, micrite matrices were studied from both shallow\ud and deeper marine settings using SEM, trace elements and carbon/oxygen isotopes;\ud the result shows the different sources; inner-mid ramp muds have a hemi-pelagic\ud source and could have been mostly sourced from high-Mg calcite benthic\ud foraminifera and red algae, and possible partial aragonite dominating; in contrast, the\ud outer ramp matrices, were sourced from plankton, are largely composed of low-Mg\ud calcite, as they are mineralogically stable. Although the exact origin would be difficult\ud to ascertain after diagenesis. From the above two different rock fabrics, two distinct\ud pore systems were identified: (1) low microporosity inner-mid ramp microfacies, it\ud was sourced from metastable precursors and were recrystallized and replaced under\ud meteoric waters, undergoing loss of primary porosity; (2) higher microporosity outer\ud ramp/basinal microfacies, composed of more stable low-Mg calcite that underwent\ud less recrystallization and retained some primary porosity.\ud The Kirkuk Group succession comprises of two shallowing upward 4th order cycle\ud within one 3rd order cycle located between two unconformable surfaces at lower and\ud upper boundaries which can be correlated to the global regression of sea level. The\ud first 4th order cycle is located at Rupelian and composed of only the Sheikh Alas\ud Formation and the second 4th order cycle is located at Chattian-Early Aquitanian\ud and composed of the Bajawan, Anah, Azkand and Ibrahim Formations. Two different\ud depositional sequences with different thicknesses were developed due to the\ud synsedimentary Khanaqin Basement Fault which cross-cuts the study area and was\ud activate during deposition.
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