oa Evidence for top Shu’aiba exposure and incision in Block 5, offshore Qatar: Regional consequences

An integrated subsurface data set has provided evidence for the presence of karst fissures and platform incision at the top of the Shu’aiba Formation in Block 5, offshore Qatar. These observations have significant implications for the Late Aptian palaeogeography and related understanding of the distribution of Late Aptian-Early Albian siliciclastics in the region. Well-log correlations show an incised-valley feature reaching a maximum depth of about 30 m. A seismic time slice at this stratigraphic level displays the geometry of the incision, which has a strongly meandering shape, is multi-storey, and reaches locally a maximum width of 8 km. Core from the incised channel shows an overall deepening up trend. The infill starts at the bottom of the valley with tidally influenced fluvial sandstone deposits, rich in organic matter and amber, followed by estuarine tidal channel and bar deposits, and, finally, tidal bar and shoreface deposits at the top. This was interpreted as a transgressive back-fill succession of a major valley system. Additional evidence for exposure comes from numerous clay and sand-filled fissures at the top of the Shu’aiba Formation. These observations show that the Shu’aiba in Block 5 was exposed before deposition of the Nahr Umr. From a palaeogeographical point of view, it means that the Bab Basin was separated from the siliciclastic sources in the northern part of the Gulf area. The presence of the large river systems does, however, provide a solution for the large-scale influx of siliciclastics during early Nahr Umr deposition.