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Abstract

Analysis for placement of a well in deep water typically begins with a thorough study of the seafloor. This is followed by a shallow geohazard report, which is used to identify zones of instability in the shallow subsurface (faulting, over-pressured zones, etc.).

An example from a 415.55 km2 (160.44 mi2) 3D seismic survey, offshore Morocco, is presented. Amplitude extraction and stratal slice maps were generated within the focus area of the Ras Tafelney 3D seismic data-set volume. Three horizons have been mapped in the subsurface to track reflection events that showed bright positive amplitudes. In the survey area, the main potential hazards appear to be active sediment pathways (gullies) and shallow sands, both of which can be the site for shallow water-flow conditions. Minor faulting is present through different stratigraphic intervals but is relatively insignificant and therefore not considered to be a potential geohazard.

Gullies and canyons are the most prominent features in the study area. They include active modern sediment pathways, which may be subject to slumps and slides and therefore may negatively impact nearby seabed structures. Older groups of buried channels that may be sand prone and/or associated with pore pressure anomalies were also mapped.

Sand-rich facies in the near seafloor sediment column are not in themselves hazards but should be characterized because of the potential for problems related to setting casing points. Sandy facies are also host to shallow water-flow conditions, shallow gas reservoirs, and hydrates.

Improvements in estimating drilling risk and costs that could be carried out include the analysis of offset logs, velocity data, sediment properties, and pressure data. In concert with the existing seismic data, these data can be used to create pore pressure cross sections and other displays that may reduce drilling risk and costs.

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