Paleozoic stratigraphic lexicon and hydrocarbon habitat of Iraq

Abstract

The crystalline Proterozoic Basement does not crop out in Iraq, but is interpreted from seismic and geophysical potential data to range in depth from about 6–10 km in western Iraq, to 12–15 km in the Zagros Mountains, in eastern Iraq. The Cambrian and Lower Ordovician sedimentary successions of Jordan and Saudi Arabia (including the Middle Cambrian Burj carbonates) are interpreted to extend into Iraq based on seismic data and regional correlations. The entire Paleozoic succssion is about 3–4 km thick. The Ordovician-Permian succession in Iraq consists of ten formations that are here described in a lexicon format. For each formation, the type and reference sections in outcrop or/and subsurface are reviewed (as defined by the original authors or herein), and further documented by including subsurface data (electrical logs and biostratigraphic studies). The Ordovician-Permian formations (and their members) are here placed chronostratigraphically according to the “Geological Time Scale GTS 2004” (and standard global Ordovician stages) and the Arabian Plate sequence stratigraphic framework. The ten formations are: (1) the Early?, Middle and Late Ordovician Khabour Formation (with from base-up seven informal members K7 to K1); (2) the Silurian Akkas Formation (with the proposed lower Hoseiba and upper Qaim members); (3 and 4) the Late Devonian Pirispiki Red Beds Formation and enclosed Chalki Volcanics; (5) the Late Devonian (Famennian) and early Carboniferous (early Tournaisian) Kaista Formation; (6) the Carboniferous (Tournaisian) Ora Formation; (7) the Carboniferous (Tournaisian-Visean) Harur Limestone Formation; (8) the Visean-Serpukhovian Raha Formation (proposed here); (9) the late Carboniferous-early Middle Permian Ga’ara Formation; and (10) the late Middle and Late Permian Chia Zairi Formation (in outcrop consisting from base-up of the informal Dariri, Satina Anhydrite and Zinnar members). The Late Devonian-early Carboniferous succession, comprising the Pirispiki, Chalki, Kaista, Ora, Harur and Raha formations, is here proposed to comprise the Khleisia Group. The Paleozoic succession of Iraq is hydrocarbon-prospective in the western part of the country, and particularly in the Western Desert near Jordan, Saudi Arabia and Syria. The source-rock component of the petroleum system consists of several potential organic-rich shales including the regionally widespread Silurian Akkas “hot shale”. In the Akkas-1 well, two hot shale units have a combined thickness of 61 m (210 ft) and total organic carbon (TOC) values that reach 16.6%. Several reservoirs and seals present exploration targets in the Western Desert of Iraq. In the Akkas field, light (specific gravity of 42° API), sweet oil and gas (no H2S) were discovered in 1993 in the Akkas and Khabour formations, respectively. The Akkas reservoir occurs in the upper Qaim Member of the Silurian Akkas Formation and consists of sandstones that have a porosity of 6.5% and permeability of 0.2 mD. The Khabour reservoir occurs in the Upper Ordovician K1–K4 members and consists of sandstones with a fracture porosity of up to 7.6% and permeability of 0.13 mD. In North Iraq the carbonates of the Permian Chia Zairi and Triassic Mirga Mir formations correlate to gas reservoirs in the Khuff Formation of Arabia and the Dalan and Kangan formations of Iran, and may therefore be prospective. Southern Iraq, along the Kuwait and Saudi Arabian border, may also be prospective; however, no wells have been drilled into the deep Paleozoic succession in this vast region.

Organic petrology of Ora formation in Akkas field, western Iraq

“…According to North Oil Company (NOC) final report; Ora/Kaista formation in Akkas-1 well are at depth interval of 1082-1464m; and in Akkas-3 well at depth interval 941-1377m. [1] considered Ora formation is at interval 1297-1402 m in Akkas-1 well. The available amounts of samples for analyses were limited, therefore, palynomorph slides, or kerogen concentrate, could not be made because these slides needs large amounts of rock sample, instead, polished sections were made for organic petrology study.…”

“…The penetrated successions in these wells range in age from Tertiary to Lower Paleozoic. Akkas-1 well is the deepest, with a total depth of 4238 m. In Akkas-1well, Ora formation composed mainly of black fissile shale with intercalation of sandstone, siltstone and dolomite streaks with pyrite and gluconite [1]. Three lithofacies were identified in Ora formation by [2,3]; these are shale, sandstone, and dolostone.…”

“…In the type locality, which located in Ora anticline in the northmost of Iraq close the border with Turkey, it consists of black micaceous and calcareous shale with olive green silty marl and lenses of detrital limestone, fine-grained sandstone occurs as intercalations through the succession; with thickness about 256 m. Lower part of the formation, which was part of Kaista formation, comprises of thin-bedded dark blue argillaceous limestones, with beds of silty shale and sandstone [4]. Details of the lithology of Ora formation are summarized in [1,2,3].…”

“…The Lower Carboniferous in Turkey is represented by Ziyarettepe Formation, which contains sandy and clayey limestones and organic matter-rich black shales which are the principal effective source rocks in the eastern Taurus region [5]. In type section and in the west (wells Akkas-1 and Khlesia-1), Ora formation is overlain conformably by Harur formation; the contact is gradational [1]. The age of Ora formation is Early Carboniferous (Tournaisian) in the type section [4,6], in Akkas -1 well [7], and Khlesia-1 well [8].…”

Calibrating Mid-Permian to Early Triassic Khuff sequences with orbital clocks

“…200 m in the Fars-Zagros region. Its upper part may correlate to the Satina Member of the Chia Zairi Formation in Iraq (van Bellen et al, 1959(van Bellen et al, -2005Al-Hadidy, 2007). communication, 2003).…”