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The Tigris-Euphrates is a continental-scale fluvial system, around 2800 km in length, which drains over 1 million km2 of SW Asia. The system originated in the Late Miocene and developed into the principal axial drainage system of the region, which follows broad regional structural features of the Mesopotamian Foreland Basin. Good preservation and outcropping of the Pliocene and Quaternary Euphrates deposits yield a viable local analogue for subsurface fluvial reservoirs in the region, and for other fluvial systems that have developed in foreland basin settings. This paper documents the first detailed study of the sedimentary characteristics of these Pliocene and Quaternary fluvial deposits along the middle reaches of the present-day Euphrates in Syria.The Euphrates fluvial system developed from small and probably short-lived isolated cut-and-fill channels in the Pliocene, characterised by abundant debrite and slump facies, through to a broad meandering system at present. The Quaternary deposits represent a braided to meandering system that was more energetic than that of the modern day Euphrates. The Quaternary facies include a dominance of gravels, pebbly sands and sands as channel associations, coupled with sands, muds and paleosols representing channel abandonment, overbank and crevasse-splay associations. Channel widths, where observed, range from 50 to 500 m, and minimum fill thicknesses range from 3 to 7 m. The combined channel-fill for stacked channels is up to 25 m thick. Lateral correlation of channel elements over at least 1 km of section indicates that rapid and extensive lateral migration has occurred. Crevasse splay lobes can be identified in the overbank deposits, with a width of 30–60 m and sand thickness of 0.5–1.5 m. The geometry, nature and dimensions of these architectural elements provide a useful analogue for subsurface reservoirs. The high-energy channel facies of the Quaternary system show very good reservoir attributes, with good correlation and connectivity. At the bed-scale there is significant heterogeneity of characteristics that would impact fluid-flow for hydrocarbon production from a subsurface reservoir. However, incomplete preservation of these Quaternary fluvial deposits at outcrop remains a challenge for accurately determining the scale of sedimentary features and also the size of the paleo-river. |
