Sedimentalogical study for the Upper Shale member in Zubair formation to enhance and comparison oil production in North Rumaila oilfield

Abstract

Zubair is the most important formation in Lower Cretaceous deposition cycle in Iraq. It consists mainly of interbedded Sandstone, Shale, Siltstone and sometimes thin beds of Carbonate. This Formation has five members in Iraq; the Upper Shale is one of these members. Upper Shale rocks can be distinguished by electrofacies and grain size analysis. The electrofacies that distinguished by Gamma-ray didn’t give the true lithology. Quartz is the main component of the Sandstone in the Upper Shale with minor percentage of Silt and clay minerals. Sand grain size ranges from very fine to medium, with roundness ranging from sub rounded to sub angular. Sandstone types are arenite and wacke. Grain size analysis presented that the sand percentage is predominant in all Rumaila oilfield with a little percentage of Silt and Clay minerals. Scanning electron microscope showed the primary and secondary porosity with diagenesis process effective of granular in South and North of Rumaila oilfield. The diagenetic processes affected of Upper Shale in different intensities. The result of these processes on primary porosity, permeability, and compaction was in different ways. Upper Shale member in North of Rumaila was covered by Silt and Clay minerals, this would affect the permeability. X-ray diffraction analysis showed that clay minerals consist mainly of Kaolinite, with few presence of Illite.