From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
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Shelf-edge depositional systems are more poorly understood than their associated shelf, slope and base of slope systems. Shelf-edge deltas (SED's) are found at the edge of the continental shelf, where differences in slope, accommodation, sediment flux and growth faulting differentiate them from shelf deltas. Three shelf edge delta types (unconfined, structurally-influenced and collapsed) are recognized based on the dominant stacking patterns observed from a recent shelf edge offshore Nigeria formed during the last glacial lowstand, and an outcrop analogue from the Eocene Central Basin in Spitsbergen.
Unconfined SED's are broadly progradational, with a moderate to highly reflective clinoform-bearing wedge. Upper slope channels emanating from the delta front suggests coeval turbidite deposition, sourced from across the delta top. Structurally-influenced SED's are strongly aggradational and are found in the hanging-wall of active growth faults. Down-lapping, high amplitude clinoforms in this SED type are localized in the sedimentary wedge adjacent to the growth fault. Structurally-influenced SED's are likely to be sand rich owing to the capture of coarse sediment along the active growth fault. Collapsed shelf-edge deltas result from delta-front over-steepening, local shelf-edge collapse or other gravitational instability, with subsequent mass failure and shelf edge retreat forming a characteristic retrogradational stacking pattern and formation of a canyon head at the shelf edge. Low amplitude, chaotic, discontinuous reflectors alternating with sinuous, higher amplitude reflectors characterise the coeval upper slope seismic facies.
SED reservoirs are locally thicker, but often less laterally extensive than shelf deltaic reservoirs . Seismic facies and architecture of the recent analogues are comparable to deeply buried exploration targets suspected of being shelf-edge deltas . Key challenges to the utility of these shallow analogues to exploration targets include the lower seismic frequencies, lower overall signal to noise and complex structural overprint characteristic of exploration depths. Despite these challenges, it is possible to identify numerous similarities between the high-resolution seismic facies observed in the analogue dataset, and the seismic facies present in the exploration targets.