Publication Date:
2003-01-01
Description:
Understanding the nature of mobile shale deformation in deltas requires appreciation of several critical factors: (1) the tectonic setting (deposition on an active or passive margin); (2) the range of potential structural styles and how to identify them on seismic reflection data; (3) how structural styles evolve with time; (4) how to apply and modify what is understood about salt deformation to mobile shale deformation; and (5) how delta structures influence sedimentation. Differences between active and passive margin settings include: (a) active margins tend to display shorter, higher gradient drainage systems associated with high rates of erosion per unit area when compared with passive margins; (b) active uplift of the hinterland will force delta progradation (not seen on passive margin); (c) uplift causes erosion and re-cycling of older deltaic deposits which can cause comparatively fine grained sandstone reservoirs; and (d) the development of large compression-related folds and thrusts in the shelfal region of deltas on active margins can create areas of active uplift. Consequently, structurally controlled embayments may develop that cause distinct changes in the dominant depositional process (wave, tidal, fluvial) along the margin. On both active and passive margins, growing gravity-driven structures locally create sea floor highs and depressions and create changes in the dip of slopes which influence depocentre location and sediment pathways from the shelf to deepwater. A wide range of mud diapir geometry can be found in deltas. Commonly mud diapirs exhibit reactive, active, passive and collapse features. Some diapirs develop as compressional features seaward of fault bounded depocentres. Also frequently associated with delta provinces are fluid pipes, shale intrusions, fluid fronts and gas clouds. All the overpressure phenomena can produce dimming, or disruption of reflections on seismic reflection data, however the characteristics of the different phenomena are sufficiently diverse that they can frequently be differentiated on good quality data.
