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Abstract

AAPG Bulletin, V. 99, No. 6 (June 2015), PP. 9851012

Copyright copy2015. The American Association of Petroleum Geologists. All rights reserved.

DOI: 10.1306/12011414036

Early synrift reservoir development on the flanks of extensional forced folds: A seismic-scale outcrop analog from the Hadahid fault system, Suez rift, Egypt

Matthew M. Lewis,1 Christopher A.-L. Jackson,2 Robert L. Gawthorpe,3 and Paul S. Whipp4

1Basins Research Group (BRG), Department of Earth Science and Engineering, Imperial College, Prince Consort Road, London SW7 2BP, UK; present address: BP Exploration, Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7LN, UK; [email protected]
2Basins Research Group (BRG), Department of Earth Science and Engineering, Imperial College, Prince Consort Road, London SW7 2BP, UK; [email protected]
3Department of Earth Science, University of Bergen, P.O. Box 7803, N-5020 Bergen, Norway; [email protected]
4Basins Research Group (BRG), Department of Earth Science and Engineering, Imperial College, Prince Consort Road, London SW7 2BP, UK; present address: Statoil ASA, Sandsliveien 90, Bergen, Norway; [email protected]

ABSTRACT

Forced folds typically develop above the tips of propagating normal faults in rifts that contain thick, prerift salt or mudstone sequences. This structural style is associated with the deposition of wedge-shaped synrift deposits that thin and onlap toward monoclinal growth folds overlying the vertically restricted fault tips. Subtle stratigraphic traps may develop on the flanks of these folds although, because of limited seismic resolution and sparse well data, the architecture, thickness, and distribution of these early synrift reservoirs are difficult to predict. To improve our understanding of early synrift reservoir development on the flanks of forced folds, we focus on seismic-scale outcrop analogs along the Hadahid fault system, Suez rift, Egypt. Our data indicate that forced folding dominated during early rifting and that the onset of folding was diachronous along strike. Fluvial systems incised the rotating monocline limbs, leading to the formation of valley-like erosional relief along the base synrift unconformity. Reservoir-prone fluvial facies are only locally developed along the forced-fold flank, with their distribution related to the degree of sediment bypass downdip into the adjacent basin. Early synrift relief not filled by fluvial strata was backfilled by transgressive, tidally influenced, reservoir-prone facies, with carbonates being locally developed in areas of low clastic sediment supply. Further extension and fault-tip propagation led to amplification of the forced folds, and deposition of shallow marine-to-shelf parasequences that became thinner toward the growing folds. Although displaying greater strike continuity than the underlying fluvial or tidal reservoirs, shoreface sandstone reservoirs amalgamate onto the flanks of the forced folds and may be absent toward the fold crest. This seismic-scale outcrop analog helps us better understand the subseismic stratigraphic architecture and facies distributions of early synrift reservoirs on the flanks of extensional forced folds. Observations from this and other well-exposed outcrop analogs should help reduce subsurface uncertainty and risk when exploring for hitherto under-explored, subtle, early synrift stratigraphic traps.

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