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Department of Geology, University of Bristol, Wills Memorial Building, Queens Road, Bristol, UK
Department of Earth Sciences, University of Leeds, Leeds, UK
A number of large, unusually shaped sandbodies have been interpreted from three-dimensional seismic data of hydrocarbon-bearing Tertiary submarine fan deposits of the North Sea. The unusual sandbody shape is considered to have resulted from post-depositional liquification of turbidite deposits. An understanding of liquification processes may therefore be important in delineating reservoir body geometry. The unusual shapes take the form of sheet-like intrusions, either along faults or as dyke and sill complexes, and domes with oversteepened sides. Three processes can cause liquification: (1) fluidization, which results from pore fluid movement; (2) liquefaction, caused by the agitation of grains during cyclic shear stress; and (3) shear liquification which results from the movement of grains during the application of a shear stress across the sandbody. In laboratory experiments each liquification process produces its own style of deformation. Fluid escape and dish structures are produced during fluidization, load structures form during liquefaction, and mass flow structures result from shear liquification. The three liquification processes can interact to create an even greater diversity in deformation style. The active liquification process, or combination of processes, can change in both space and time. In the natural environment it is unlikely that one liquification process will occur independently and therefore most natural liquification must be considered in terms of two or more interactive processes. The large-scale deformation of sandbodies and hence their geometry may vary according to which liquification process or processes are active. In natural systems the sandbody will not deform in isolation from the surrounding material. The rheology of the surrounding material and the nature of the stress field active at the time of deformation will play an important part in controlling the behaviour of the sandbody during remobilization.
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