Abstract
The upper sandstone member (Z/2) is one of the most important reservoirs in the Zubair Oilfield, south of Iraq. It belongs to the upper part of the Zubair Formation, the Lower Cretaceous age, and deposited in deltaic environments. The upper sandstone member consists of several sandstone units which are isolated by the shale units. Major trace and rare earth elements (REEs) of the upper sandstone member in Zubair Formation have been analyzed. Six core samples were used to determine the source of sediments, paleo-environment redox condition and elements concentrations. The geochemical analysis data indicate that the source of sediments rock was influenced by a low to moderate degree of weathering conditions in the Hammar and Safwan Domes. The results have shown that all samples were formed in an oxic environment, with humid paleoclimate. Geochemical analysis data indicate that all rock samples are fundamentally derived from rhyolitic and andesitic rocks more a felsic source. The results show that the samples are mainly of marine organic matters input. The values of Chemical Index of Alteration in the Hammar Dome range from 13.05 to 24.37, while from 23.72 to 77.22 in the Safwan Dome. These results may attribute to fine and medium clastic sedimentary rocks in the H sandstone unit, with a low degree of weathering in the Hammar Dome, and high to medium with slightly higher abundance of clayey silts in the Safwan dome.
Highlights
The chemical compositions of the sedimentary rocks are the products of many factors such asIraqi Geological Journal Al-Jaberi and Al-Jafar 53 (1E), 2020: 55-74 transport, weathering conditions, provenance, diagenesis and climate (Johnsson and Basu, 1993)
The rare earth elements (REEs) were normalized according to the values of Chondrite according to McDonough and Sun (1995). The chemical indicators, such as chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), and Parker Weather Index (WIP) were used to determine the intensity of weathering (Parker, 1970). This is further supported by calculation of the Chemical Index of Weathering (CIW) (Harnois, 1988), Plagioclase Index of Alteration (PIA) and Index of Compositional Variability (ICV) (Cox et al, 1995)
A significant ratio of light rare earth elements (LREE) gives an indicator to lack of ferromanganese and heavy minerals in the study area, low LREE/heavy rare earth elements (HREE) ratios suggested that fine-grained of sand with clay content in the Safwan Dome as a result present of clay minerals; kaolinite and illite
Summary
The chemical compositions of the sedimentary rocks are the products of many factors such asIraqi Geological Journal Al-Jaberi and Al-Jafar 53 (1E), 2020: 55-74 transport, weathering conditions, provenance, diagenesis and climate (Johnsson and Basu, 1993). The chemical compositions of the sedimentary rocks are the products of many factors such as. Geochemical analysis of major and trace elements can provide a detail of the sediment composition, origin of the sedimentary rocks, and paleo-weathering conditions (Nesbitt and Young, 1982; Taylor and McLennan, 1985). Zubair Oilfield is located in southern Iraq, approximately, 20 Km southwest of Basrah city. It is located between 47o 32`– 47o45`Latitude and 30o 42`–30o 05`Longitude. It covers an area of about 1170 km2 It is bounded from the north by the Nahran Umr Oilfield, and from the west by the Rumaila Oilfield and from the south by the Kuwait-Iraq borders (Fig.). The Zubair Oilfield consists of three domes, Safwan, Rafidiyah and Shuaiba-Hammar Domes from the south to the north respectively (ZFOD, 2016) (Fig. 1)
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