Unconformities, Burial and Uplift in the Western Desert of Iraq: Magnitude of Stratigraphic Breaks and Implications for Hydrocarbon Exploration

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This report focuses on the effect of unconformities in the shallower (Upper Palaeozoic-Cenozoic) stratigraphy on pre- and post-Hercynian maturation of the deeper (Lower Palaeozoic) stratigraphy. Of principal interest are the shallow stratigraphic penetrations (e.g. water wells) that contain Mesozoic/Cenozoic stratigraphic information that may calibrate post-Triassic deformation within Iraq, and data from neighbouring areas (principally Saudi Arabia but also including Jordan and Syria as necessary).

Very few significant unconformities actually developed within the Western Desert of Iraq and the stratigraphy appears to be relatively complete, when compared to adjacent regions. Most of the major breaks lie just outside of the study area and gave rise to very positive structures, the timings of which are all different. These uplifts include a Taurus-Zagros Uplift of Northern Iraq and SE Turkey, which is a later Devonian event; the Helez Geanticline of the Levant and its probable correlative the Western part of the Pre Unayzah Unconformity of Saudi Arabia, which principally formed as an intra-Viséan uplift that was locally re-activated in the Late Westphalian; the main Pre Unayzah Unconformity of Eastern Arabia which formed the Ghawar, Summan, Burgan highs and the Salman Zone anticlines of Iraq, during the later Westphalian; and lastly the Hamad Uplift of Syria, which formed during the later Permian and Early Triassic.

Younger Mesozoic elevation of the Western Desert area was mostly a consequence of inversion of the very thick Palaeozoic basin, and resulted in pulses of uplift whose maximum expression was in the west of the Western Desert, followed by prolonged periods of westerly onlap (e.g. during the Early-Middle Cretaceous) onto the tilted erosional surfaces. Within the stratigraphy are identified periods when there was most probably little or no deposition over the study area, although these do not necessarily indicate strong uplift and erosion; these lacunae include the Devonian, the Late Viséan-Early Westphalian; the latest Permian and Early Triassic; probably much of the Middle-Late Jurassic, and Early Cretaceous.

In terms of thermal history, most shallow wells in the western part of the study area indicate maturities that are too high for the present day burial depths, and maybe an overall uplift of 1 to 1.5km, decreasing eastwards. The Akkas-1 well in the NE of the study area appears to have a relatively stable maturity profile with the whole stratigraphy being in thermal equilibrium with present-day borehole temperatures.