Composition Characteristics of Middle-Upper Cretaceous Shoal Facies Limestone Grains and their Implications for Sedimentary Environment in Iraq Area
Shoal limestone reservoirs are widely developed in the Cretaceous of Iraq,which are difficult to divide and predict because of the complex grain types of these reservoirs.Previous studies have shown that the Middle and Upper Cretaceous in Iraq mainly developed spheroids,green algae and echinoderms,followed by foraminifera,bonin and endoclast.It is helpful to further clarify the sedimentary microfacies and reservoir types to indicate the sedimentary environment of the middle and Upper Cretaceous.Using core and wafer observations,different types of particles were identified and the grain types of Kh2,Mi4 and Mal layers of the Middle Upper Cretaceous were defined.Green algae and thin-walled lamellibranchias reflect the environment of normal salinity sea water or shallow sea with wide salinity and relatively limited water energy.Among foraminifera,bonin is a mobile benthos,which is widely distributed and reflects strong hydrodynamics.The thick-walled lamellibranchias and endoclasts were formed in the high energy environment of shallow sea.Echinoderms are not selective to sedimentary environment and have a wide distribution range,which can be seen in every horizon.The understanding of different grain types of typical strata of middle and Upper Cretaceous provides petrographic basis for oil and gas development and reservoir research in Iraq.
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