Inorganic geochemical evidence for the depositiona

Authors: Ibrahim M J Mohialdeen Sardar M Raza

Publish Date: 2012/10/30

Volume: 6, Issue: 12, Pages: 4755-4770

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Abstract

Four wells K109 Hr1 Tk3 and Bj1 in NE Iraq including parts of the Kurdistan Region were selected to study the Tithonian–Berriasian Chia Gara Formation from the inorganic geochemical point of view The intervals studied in each well occurred at presentday burial depths of Hr1 3075–3310 m K109 2780–3090 Tk1 2770–2890 and Bj1 2150–2310 m A total of 16 samples from the four studied wells were investigated geochemically using Xray fluorescence in order to measure their major element oxides and their trace element contents Among the major oxides CaO has the highest weight percentages in all samples as expected in this limestonedominated formation SiO2 and Al2O3 show higher concentrations only in well Bj1 than the other sections due to its shallower depth of deposition and its marginal location within the depositional basin The general trace element distribution along all studied well successions showed good similarity However the ratios of V/Ni and V/Cr were of higher values in the lower part of the formation which is considered as a good indicator to the deposition of this part of the formation within anoxic depositional environment An exception was in well Bj1 where the V/Ni ratio was lower in the this lower part of the formation than the upper part that may be due to the different lithology in this succession which is lacking distinct shale or calcareous shale beds However the V/Cr ratio can still indicate the prevailing of reducing condition because V/Cr ratios in all samples are 2 Also Th/U ratio in the lower part of the studied successions was lower than the upper part which is also coinciding with the domination of the reducing geochemical conditions in the depositional environment It may be concluded from this study that the Rmode cluster analysis of the main oxides indicated to the main mineral constituent of the rock which is calcite It is also confirmed that the SiO2 and Al2O3 contents in the Chia Gara limestones are especially high in well Bj1 particularly in the lower and upper parts of the succession On the other hand Rmode cluster analysis of the trace elements showed lower amounts of Zr and V in relation to silica and confirmed the random distribution of Sr and Ba in the studied samples Qmode cluster analysis indicated that the upper part of the studied formation is pure limestone in well H1 and such purity of limestone beds also occurs in the lower part of the studied formation in well Tk3 The elemental distribution within the sections as inferred from the studied samples indicates that the anoxic depositional conditions were prevailed during the Chia Gara Formation especially during the accumulation of the lower partWe would like to thank the North Oil Company of Iraq for supplying the samples and thin sections of the studied wells The authors also would like to thank the Washington State University particularly Mr Rick Connery for XRF analyzing the selected samples and for his fruitful discussions Special thanks are offered to the reviewers for their careful and useful comments that improved the revised manuscript

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