Basin Analysis of Paleozoic Succession, Iraq

20-02-2014

Ph.D Thesis

Nawrast S. Abd Alwahab

Abstract

   The Paleozoic basins of Iraq are analyzed from Upper Ordovician to lowermost Triassic (Induan) sequences. The deepest wells in Iraq that bottomed to Paleozoic successions are covered in this study, these are: Akkas-1,2,3; Khleisia-1; KH5/1; West Kifil-1; Diwan-1; Mityaha-1; Atshan-1 and Jabal Kand-1 wells. The sequence stratigraphic models of Arabian Plate are applied to these successions. The first five tectonomegasequence (TMS AP1-AP5) and the lower supersequence of AP6 are recognized and correlated regionally. Both of TMS AP1 and lower part of AP2 are neither exposed nor drilled in Iraq, so only a predicated review are presented of these megasequences.

   The sequence stratigraphy analysis shows that the Middle to Upper Ordovician sequences of Khabour Formation in western Iraq are composed of two large depositional sequences (DS O30 and DS O40). The Late Ashgill glaciations event of Arabian Plate has impact on western Iraq as falling- stage system tract represented the uppermost part of Khabour Formation. This system tract is based by a base surface of force regressive, and topped by correlative conformity surface. However, this system represented the lower part of tectonomegasequence AP3; and can be separated from Khabour Formation as a unique depositional sequence and named as new formation that equivalent to Sarah Formation in Saudi Arabia and Traibeel Formation in Jordan. The previously so called ‘Upper Ashgill unconformity’ is expressed as base surface of force regressive below the falling- stage system tract of uppermost Khabour Formation. The lowstand normal regression is not recognized in the lower Paleozoic succession in the studied area of Akkas and Khleisia-1 area.

   The maximum flooding surface S10 is interpreted to be within the black hot shale. Alternatively, the use of stacking pattern technique in this study suggested different views of black shale transgressive system. The development of high order maximum flooding surface S15 is locally in Iraq and Syria; and has been proved by general stacking pattern changes. The regional prograding system of S10 is interpreted and named in the present study as S15 highstand system tract. The latest Silurian (Pridoli) and Early to Middle Devonian are not proven to be in western Iraq, it is suggested that the Mid Paleozoic “Upper Silurian- Upper Devonian” hiatus is older in Iraq.

   Unlike the whole stratigraphy of the Arabian Plate AP4 tectonomegasequence; Iraq has a complete sequence of this TMS. The Khleisia Group of Upper Devonian- Lower Carboniferous is subdivided into two third order sequences. The MFS D30 is revised to be within Kaista Formation rather than Ora Formation, and the MFS C10 is revised to be within (Bir El) Rah Formation rather than Harur Formation. Additionally, three other maximum flooding surfaces of higher order (C5, C6 and C7) have been recognized in study area. The depositional sequence C5 is of spatial type that has been controlled by regional tectonsim and sea-level variations. While the depositional sequences C6, C7 are of reciprocal sedimentation type.

   It is revealed that the tectonomegasequenc AP5 is represented only by the Ga’ara Formation. The sequences of this megasequence are composited of many different parts of Iraq. The base (Mid Carboniferous) unconformity was determined only in KH5/1 well, followed by sequences of Lower Ga’ara at KH5/1 well, Middle Ga’ara at shallow boreholes, and Upper Ga’ara at West Kifil-1, whereas, the top (early Upper Permian) unconformity was determined only in West Kifil-1 well. However, both of the environmental and palynological changeovers of glacial phase of the Late Carboniferous and deglacial phase of Early Permian are recognized in lower and middle members of Ga’ara Formation.

   The present study has considered the Ga’ara sections in northern Iraq (i.e. at Jabal Kand-1) as the basal clastics of Chia Zairi Formation, while the part of Upper Ga’ara in Atshan-1 has been considered as upper mixed clastic unit of Chia Zairi Formation. Alternatively, three independent criteria are provided to distinguish between the clastics of the Upper Ga’ara Member and the clastics of the Basal Chia Zairi unit because they refer to different stages of basin formation. However, the depositional sequences of Chia Zairi Formation, in the studied area, are quite similar in its lower and middle parts to the equivalent Khuff Formation in the Arabian Plate, while the upper part is more affected by clastic sediments, especially in its northern subsurface sections.

   The quantitative subsidence analysis is performed to the Paleozoic basins by applied 1D backstripping technique. Although, the diagnostic signature of continental stretching is clearly observed from the backstripping results of Akkas-1 and Khleisia-1 wells; but it cannot simplify these results to infer the subsidence mechanisms that formed these basins. The basin infilled models revealed that the Paleozoic basins in Iraq have been evaluated through three rifting phases: Upper Ordovician- Lower Silurian phase, Upper Devonian- Lower Carboniferous phase and Upper Carboniferous - Upper Permian phase. The first two phases are interrupted by inversion phases, which in turn terminated the development of their rifting processes. The stratigraphic architecture of initial, underfilled, rift climax and postrift for the different stages of these rifting phases have also inferred.

   It is revealed that, the general geodynamic framework of the Paleozoic basin in Iraq is its setting as intraplate basins, which exhibit extensional driving mechanisms. These basins are “superimposed failed rift basins” that had undergone multiphase history of subsidence and uplift. Some of these subsidence phases are typically of synrift and postrift stage of rift basin evolution. Nevertheless, it is clearly that most of these rift events had not successfully developed from intraplate basin to passive margin setting till the Late Permian.

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