Petrography and geochemistry of the Late Devonian Pirispiki Formation in northern Iraq: Implications for Reservoir Characteristics and reconstruction of depositional environments

Petrographic, mineralogical and whole-rock geochemical investigation of siliciclastic and interbedded carbonate rocks from the Devonian Pirispiki Formation in the Nazdur and Ora sections, extreme northern Iraq, were conducted to assess reservoir potential and to study the effects of Devonian deformation (Middle Paleozoic, Caledonian event) on paleogeography and sediment transport in northern Gondwana. Traditional petrographic and scanning electron microscopy, X-ray diffraction and X-ray fluorescence were used. The main results are: sandstones from the Ora section contain significantly more quartz and less lithic fragments than those from the Nazdur section. This variation in sandstones composition may reflect uplifting effect due to Middle Paleozoic event and/or different provenance. The most notable feature of the major and trace elements distribution in the studied mudstones and sandstones is that they are significantly enriched in U but significantly depleted in Zr and Hf. The relative absence of zircon in the samples could account for the depletion of Zr and Hf, two elements linked to zircon. The extremely high U concentrations seen in mudstones and sandstones may relates to precipitation by circulating groundwater where it transported into the sediment prior to cementation. No fauna or flora have previously been reported from the Pirispiki Formation at outcrop in Iraq. The widespread occurrence of shell fragments in sandstones from the Nazdur section combined with the presence of eight dolomite and dolomitic marl beds suggest that the lower and middle parts of this section were deposited in a marine environment. This has not previously been recognized. Previous correlations between the Chalki basalts in the Chia Zinnar anticline and conglomerates in the Ora anticline are not supported by our data. We propose a new correlation between the sections in northern Iraq and those from southern Turkey and western Iraq that suggests that the Pirispiki Formation was deposited during a relative sea-level fall. The paleo-shoreline advanced northward through the study area during the Late Devonian. The Pirispiki Formation has been proposed as a potential reservoir for hydrocarbons sourced in underlying Paleozoic units. However, the sandstones and carbonates investigated during this study have no potential as a reservoir due to pervasive carbonate cements.