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Deep Marine Sequences |
1 Mobil Exploration and Producing Technical Center PO Box 650232, Dallas, Texas 75265-0232, USA
2 Mobil Exploration and Producing U.S. Inc., PO Box 650232, Dallas, Texas 75265-0232, USA
3 Earth Resource and Environment Center The University of Texas at Arlington PO Box 19049, Arlington, Texas 76019, USA
4 Mobil North Sea Ltd, 3 Clements Inn, London WC2A 2EB, UK
5 Mobil New Business Development, PO Box 650232, Dallas, Texas 75265-0232, USA
6 Mobil Exploration Norway Inc., PO Box 510, 4001 Stavanger, Norway
The sequence-stratigraphical concept of the basin-floor fan within the Vail/Exxon conceptual sea-level model is popular in hydrocarbon exploration because this model predicts that basin-floor fans are composed of sand-rich turbidites with laterally extensive reservoir geometries. Many producing deep-marine facies in the Palaeogene and Cretaceous of the North Sea have been interpreted as basin-floor fans based on their seismic (e.g. mounded forms) and wire-line log signatures (e.g. blocky log motifs). We have examined nearly 3700 m (12000 feet) of conventional core from features interpreted as basin-floor fans in the North Sea and adjacent regions to determine the sedimentary facies, which actually comprise these features. Our study reveals that basin-floor fans are predominantly composed of mass-transport deposits (mainly slumps and debris flows); turbidites are extremely rare. Sedimentary features indicative of slump and debris-flow origin include sand units with sharp upper contacts, slump folds, discordant, steeply dipping layers (up to 60°), glide planes, shear zones, brecciated clasts, clastic injections, floating mudstone clasts, planar clast fabric, inverse size grading, and moderate to high matrix content (5–30%). Calibration of these facies in long cores (120–210 m or 400–700 ft) with seismic and wire-line log signatures suggests that seismic mounds and blocky log motifs are actually a manifestation of amalgamated sand units emplaced by these mass-transport processes. The cores show that in some cases, sequence boundaries on seismic profiles actually correspond to primary glide planes (decollement) between slump sheets in cores examined. Unlike laterally continuous turbidites, slump- and debris-flow-emplaced sands are often laterally discontinuous, thus making reservoir geometry and continuity much harder to predict. Nevertheless, these mass-transport deposits still form important hydrocarbon-producing reservoirs (e.g. the Frigg Field). Because both sands and muds deposited by slumps and debris flows form features that are routinely interpreted as basin-floor fans on seismic and log data, process-sedimentological interpretation of conventional core is critical to the establishment of the true origin and distribution of sands contained in such features and thereby evaluate their hydrocarbon potential.
