Reservoir characterizations of the upper cretaceous deep marine turbidites to the close northern part of Sulaimaniyah, (Northern Iraq)

In this study reservoir characterization of the Upper Cretaceous low density turbidite sandstones of Tanjero Formation, northwestern part of Sulaimaniyah city, Northern Iraq, were analyzed. The sandstone portion of the unit have been examined through field and laboratory based studies. Seven logs were measured and described in detail. Average thickness of the measured sections is 158 m. The field logs start from the contact between underlying Shiranish Formation to more than 150 meters in thickness in the turbidite unit. 44 rock samples were taken for petrographic analysis, porosity and permeability tests from the logs for laboratory analyses. Seven distinctive main lithofacies groups consisting of forty-eight subfacies were identified from seven measured sections for facies analysis. The groups classified as grain size are coarse grain sandstone beds (CGSB: includes four subfacies), medium grain sandstone beds (MGSB: includes twelve subfacies), fine grain sandstone beds (FGSB: includes eighteen subfacies), very fine grain sandstone beds (VFGSB: includes two subfacies), shale and silt sized thin bedded turbidites (STBT: includes five subfacies), fine sand thin bedded turbidites (FSTBT: includes four subfacies) and medium sand thin bedded turbidites (MSTBT: includes three subfacies). The subdivisions of the groups are separated according to their internal sedimentary structures. All facies groups represent low density turbidites composed of different divisions of the Bouma sequence. These facies represent about middle part of outer fan and lobe fringe to lobe distal fringe sand rich depositional system. Petrographic analysis points out that, the sandstones are calclithite (litharenite), very fine to medium grained in size consisting of chert, siltstone, mudstone, radiolarian chert and radiolarian mudstone fragments, angular to subangular in shape, very poorly to moderately sorted, transported over short distances and represent submature stage. Mainly long grain, concavo convex and sutured grain contact types and high contact index (4.7) represent moderate to tightly packing, moderate compaction which are negatively III effecting the reservoir quality. Lithic recycle category obtained from petrography is the only tectonic provenance composed of sedimentary rocks of the Lower Cretaceous Qulqula Formation for the sandstones. Porosity of the studied sandstones range from 5.51% to 11.59 % which is poor to beginning of fair with an average value of 7.11%. The obtained permeability varies between 10.71-2315.65 md and their mean value is 866.35 md representing high permeability. As the modal components of the sandstones are made up of 99.621 % reworked sedimentary rock fragments with high carbonate rock clasts, and existence of calcite minerals, the reservoir quality of the turbidite sandstones was strongly negatively influenced by the alteration of these elements creating mainly calcite cementations. The effect of calcite occurrences and solid bitumen fillings ranging between 1-15 percentage plugging of pores cause a final reduced porosity. As a result, the turbidite sandstones of the Tanjero Formation cropping out to the northwest of Sulaimaniyah have general poor porosity and a very good permeability. Keywords: Tanjero Formation, turbidite sandstone, reservoir characterization, northwestern Sulaimaniyah, Northern Iraq