Mishrif Formation (Late Cenomanian) has been studied in four oilfields in southern Iraq, and studied samples were examined by mineralogy and geochemistry analysis using XRF and XRD techniques. The mineralogical study reveals that dolomite and calcite are the main minerals in the rock formation in studied oil fields, whereas quartz and clay minerals (kaolinite, montmorillonite and illite) represent secondary minerals composition. For geochemical classification of Mishrif rock, Ca/Mg ratio was applied and this reveals that Calcitic Limestone is a major type in studied samples. The paleo - salinity has been examined by using Ca/Mg and Mg/Ca ratios and they referred to moderate salinity during deposition. Mg/Ca and Mn/Ca ratios have been applied to measure paleo-temperature, so infer a moderate paleo-temperature for Mishrif Formation in studied samples. Based on the comparison between low Sr/Ca and the relatively high Ca/Mg ratios, infer that a dolomitization process played the main role among diagenesis processes.   http://dx.doi.org/10.25130/tjps.25.2020.013

Stable isotopes 18O/16O and 13C/12C in the carbonate rocks of the Mishrif Formation are examined here to define the depositional characters in the basin includes paleo temperatures and paleo depth.      The Mishrif formation (Cenomanian – Early Turonian) has extensive distribution in Iraq and Middle East. Mishrif Formation composed of organic detrital limestone. Four boreholes in four oilfields, Noor – well (11), Amarah – well (14), Buzurgan – well (24), Halfaya – well (8), in south east of Iraq have been studied. The studied samples have negative δ18O isotope values studied well, with Average (-4.11‰), (-4.47‰), (-4.48‰), (-4.18‰) in the studied wells respectively. Carbon Isotope composition positive values in the study samples, with average (2.32‰), (1.84‰), (2.20‰), (2.24‰)  are studied well respectively. The Stable isotope (18O/16O and 13C/12C) of Mishrif Formation provide the first records of      paleotemperature for the (Cenomanian – Early turanian ) which indicate moderate  during deposition.      The plot of depositional environment refers to warm water carbonate sediment and the bulk samples cluster tightly to small positive reflect modal isotope composition of dominant skeletal marine carbonate. The paleo-salinity (Z values) provide the marine deposits in studied samples. As the paleo-temperature was high temperature by (T values). The paleo depth he all studied samples in the four oilfields are fall in the same level which is in the upper part of Thermocline this indicate the moderate depth