3D seismic expression of km-scale fluid escape pipes from offshore Namibia


J.L. Moss, School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3YE, UK. E-mail: mossjl@cardiff.ac.uk


This paper documents a large number of large km-scale fluid escape pipes with complex seismic expression and a diatreme-like geometry from the mapping of a 3D seismic survey, offshore Namibia. These pipes are crudely cylindrical, but occasionally have steep conical geometry either narrowing upwards or downwards. They are generally ovoid in planform and their ellipticity varies with pipe height. Vertical dimensions range from ca. 100 to >1000 m and diameters range between 50 and 600 m. The lower part of the typical pipe is characterised by a sag-like or collapse type of structure, but this is only imaged well in the wider pipes. The upper part of the typical pipe is characterised by gently concave upwards reflections, with a negative relief of tens of metres. There is some evidence (pipe cross-section geometrical variations and amplitude anomalies) that these concave upwards reflections are vertically stacked palaeo-pockmarks. A conceptual model for pipe formation is proposed that involves hydraulic fracturing and localisation of focused vertical fluid escape with volume loss at the base of the pipe inducing collapse within the pipe. Continuing episodic fluid migration through the pipe produces further collapsing of the core of the pipe and pockmark structures at the top of the pipe. Longer term seepage through pipes is manifested in zones of amplification of reflections above the top of the pipe.