The Role of Geological Setting and Depositional History in Offshore Slope Instability

Abstract

Abstract Submarine slides pose the most critical offshore geohazard. Seafloor mass movements occur in continental margins worldwide, and at all scales, from the smallest ones, barely detectable by acoustic methods, to the gigantic Storegga Slide off Mid-Norway. As most deepwater petroleum resources are located in continental slope settings, seafloor instability is a major concern, not forgetting that other geohazards, such as shallow gas, gas hydrates, and mud volcanism also must be assessed. The slope angles in general are small, so the most important factors controlling stability are sediment types and the presence of excess pore pressure. Both are functions of geological setting and depositional regime and history. Two important causes of excess pore pressure in the uppermost sediment layers are high sedimentation rates combined with unfavourable layering, and the trapping and accumulation of gas seeping from deeper levels. Along the high latitude, formerly glaciated NW European margin, the climatically induced variability between glacial and interglacial situations seems to have been the main controlling factor for instability. Excess pore pressures were created in stacked interglacial and glacial units during rapid burial by glacial debris during peak glacial periods. The water source may have been underlying biogenic oozes. Mineralogical factors, such as smectite content, appear to have limited influence here. The major, large scale sliding has also formed seafloor topography with numerous steep local escarpments from which smaller slides may threaten seafloor installations. In lower latitude, river-fed settings the mechanisms are different. In the Krishna-Godavari River delta off East India, the sedimentation rate varies greatly laterally and temporally, often at short time scales. The temporal variations are caused by the long term climatic variations as well as by the seasonal monsoons. In the lower latitude continental margins significant sea level low-stands, such as during the Last Glacial Maximum (LGM), may have caused exposure of the shelf, with erosion and delivery of river sediments directly to the upper slope. Small mass movements (but still large enough to be a significant threat to installations) seems to occur frequently in this setting, partly supported by locally extreme sedimentation rates. Introduction Offshore geohazards comprise a number of geological phenomena, such as submarine slides, shallow gas and dissociation of gas hydrates, shallow water flow, mud volcanism, and seismicity. Of these, the most serious is probably submarine sliding, which not only has an immediate effect on any seafloor installation in and downstream of the slide scar, but also may have serious third-party consequences if the slide has tsunamigenic potential. Hydrocarbon exploration and exploitation have, over the last decade moved into increasingly deeper waters of the world's continental margins. This also means it has moved into increasingly more slide-prone areas. At the same time, the tools for detailed seafloor investigations have developed immensely, and new areas are mapped at great detail. This again has led to the discovery of submarine slides of various sizes in all margin areas of the world.