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The distribution of traps and reservoir rocks in extensional basins is a result of the basin configuration, implying that it varies during the development of the basin. The basin configuration is different in the pre-rift, syn-rift and post-rift stages, and is a response of the interplay between structural-thermal and sedimentation processes.
Generally, the pre-rift stage is either characterized by a wide and shallow area of deposition or by the development of a dome. In both cases the relief is relatively modest, and hence, so is the relief across faults. In the syn-rift stage extensional faulting accelerates and the basin relief increases accordingly. This opens for the development of a variety of trap types, structural and subtle, and a complex distributary system for sediments. Finally, the post-rift stage is dominated by infill of the syn-rift basin, accelerated subsidence of its central parts due to thermal cooling and sediment loading and compaction. In this process, the relief becomes smoothened when the basin is filled by sediments and water. The transition from syn-rift to post-rift development coincides with the point in time when leakage of heat becomes dominant over the heat input, that is due to lithospheric thinning in the syn-rift stage. This pint in the basin development is manifested by a switch in the mode of rotation of sediment strata from being away from the basin axis to being towards it. In this situation, sediment transport and reservoir distribution again follow a simpler pattern and the variance in trap types becomes less.
Most basins, however, display large-scale configuration, complexity of traps and reservoir distributions that deviate from this simple scheme. This may be due to the basal mechanism which initiate the basin formation ("active" or "passive" extension), structural heterogeneities in the extending lithosphere, diverse deep fault geometry and fault-linkage resulting in segmentation of the basin, development of complex transfer faults, and varying extension rates and extension factors along the basin axis. In large basin systems, stress fluctuations and stress inhomogeneity also seem to be common. These complications are sometimes also reflected in a polychronous development, implying that e.g. the switch from the syn-rift to post-rift stage is not simultaneous throughout the basin system.
Due to the existence of a large database of excellent quality and a relatively mature stage of exploration, many complexities of the types mentioned can be demonstrated in the Viking Graben of the northern North Sea. Still, we are not yet at a stage of analysis where all can be explained.
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