Mass Transport Deposits:Implications for Reservoir Seals

This thesis uses a combination of two-dimensional (2D) and three-dimensional (3D) seismicreflection data to investigate the morphology and internal architecture of mass transport deposits (MTDs) from the west Nile delta, offshore Egypt and from the Deepwater Taranaki Basin, New Zealand. The overall aim of the project is to gain an improved understanding of the impact of MTDs on hydrocarbon seals. To this end a novel seismic based classification for MTDs that is directly mappable onto the problem of defining their potential as sealing sequences is proposed based on results of investigations from three core research chapters. In the first core chapter, the phases and mechanisms of failure of three main MTDs (termed A, B and C) in the western Nile delta (Eastern Mediterranean) was investigated using 3D seismic data. Analysis of the geometries of the MTDs suggests that they were emplaced in at least two main phases: Progressive failure occurred initially with MTD A cutting through MTDs B and C. Retrogressive failure then occurred due to the natural unbalance of the supposed headwall of MTD A. Reconstruction of the pre-seabed topography suggests a minimum total volume (residual and depleted) of remobilisation of the order of 750 km3, making this amongst the largest submarine landslide complexes documented to date. It is suggested that this giant slope failure might have posed a risk to the integrity of petroleum seals considering that the failure occurred in a gas rich province of the western Nile delta. In the second core chapter, six large scale MTDs, stacked and locally amalgamated making up c. 50% of the stratigraphic succession were recognised and mapped using 2D seismic data. Key kinematic features suggest a north-westerly transport direction for all of the MTDs with the exception of MTD 6, having a south-westerly transport direction. There is seismic stratigraphic evidence in the form of both lateral and basal truncations suggesting that the MTDs were erosive during their emplacement. Estimation shows that c 70% of the final volume of MTD 6 was contributed from substrate cannibalization greater than the other five MTDs in the study area. This work suggests that the rheology of the substrate immediately beneath the seafloor might not be the only factor controlling the degree of substrate cannibalisation during MTD emplacement and that the nature of any triggering mechanism might also play an important role. This has a bearing on the MTD content (sandy or muddy dominated MTD) and would in turn influence the seal integrity of MTD. The final core chapter presents insights into the dynamics of emplacement of MTD 6 using 3D seismic data. MTD 6 consists of five distinctive domains (labelled A – E). Domains A – C show evidence of deformation and remobilisation of c. 30% of a lower transparent interval while Domain D is characterised by a partial or complete loss of seismic character coupled with c. 30% reduction in thickness compared to adjacent domains. Based on the deformation styles, the transition across domain boundaries and the observed volume loss, a progressive stratal disaggregation is inextricably linked with gravity spreading as the mode of emplacement of MTD 6. This study highlights the power of 3D seismic data in unravelling the detailed processes involved during MTD emplacement and which may have significant implications as regards the translation and emplacement of other submarine slope failures in other continental margins.