Reservoir Analogues: A Small Scale Polygonal Fault System Study

Traditionally, one of the most common approaches for looking at geological structures of oil and gas reservoirs is through outcrop analogues. However outcrops are limited on detail based upon the level of their exposures. Enhanced high resolution imaging of continental shelves now enables more detailed imaging of comparable bedrock sections where they are exposed at or near the seabed. This work is focussed on such exposures of Polygonal fault systems (PFS) within the London Clay of the Outer Thames Estuary. Currently the majority of our information on PFS is from 3D seismic imaging. Modern conventional 3D seismics are capable of imaging features as small as decametres bins with metres of vertical resolution which while perfect for conventional PFS with faults within the networks extending for hundreds of metres to kilometres and throws of tens of metres there is the probability of missing smaller-scale faults within the network. An example these smaller faults in PFS networks has been identified in the Outer Thames Estuary, here the London Clay bedrock presents at the seafloor with a polygonal network in planform. By using 1m resolution bathymetry data in conjunction with a series of high resolution grid of 2D seismic lines over 1000 surface expressions of faults have been identified extending for 10s-100s of metres with what appears to be displacements of decimetres to metres with seismic lines covering a few cells of this network showing this. There are no preferred orientations for these faults and while these could represent extremely large scale thermal desiccation cracks, the presence of the London clay bedrock at the surface indicates that these are not the result of soil processes. Furthermore the London Clay is known to continue across the southern North Sea and outcrops in Belgium as the Ieper Clay, the same formation where PFS where first discovered. All this likely indicates that we have identified a smaller scale variation of PFS that have previously been hidden by the resolution gap of conventional seismic imaging.