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3-D Seismic Characterization of Sand Injectites in the Palaeogene Succession of the Northern North Sea

Abstract

High resolution three-dimensional seismic data, integrated with well data have been used to investigate the occurrence of sand injectites (or intrusions) within the Paleogene succession of the northern North Sea Basin. These sand injectites occur as discordant high amplitude anomalies which are distributed within three Cenozoic sequences (CSU-1: Paleocene to Early Eocene; CSU-2: Eocene and CSU-4: Oligocene) and exhibit varying simple to complex geometries in cross section. The discordant amplitude anomalies occur predominantly as either isolated discrete anomalies (about 70%) characterized by circular/sub-circular or elliptical form in plan-view or as amalgamated and stacked complexes (about 30%) characterized by irregular three-dimensional cross section geometry and plan-view form. They are also generally characterized by an even high amplitude through much of their extent and comprise of both top and base reflection which are often roughly parallel with varying continuity. The discordant amplitude anomalies are distributed mainly in the eastern, north-eastern, western and south-western parts of the study area, and vary in geometry from conical shaped, wing-like or saucer shaped to highly irregular and complex shaped anomalies with the conical shaped anomalies being the most predominant. Well calibration of the observed discordant amplitude anomalies intersected by wells (in Block 30, 34 & 35) at different depth and interval show the presence of 10 - 95m of sandstone (from the gamma-ray log) encased within mudstone-dominated succession, with the conical shaped intrusions believed to represent injected sand bodies while the saucer-shaped or wing-like intrusions may likely represent initial depositional sand bodies which were subsequently subjected to remobilization and intrusion. Measurements of the geometrical parameters for the conical and saucer-shaped intrusions within the CSU-1 (44 intrusions), CSU-2 (58 intrusions) and CSU-4 (65 intrusions) Cenozoic seismic sequence have been carried out on the best resolved intrusions and the measured parameters range from: 2 - 300 for the dip of discordant limbs or wings; 24 - 226m for the height or vertical extent of discordant limbs or wings from the base of the intrusion; 239 - 3264m for the top diameter (or upper width/maximum lateral extent) of the intrusions; and 41 - 2336m for the base diameter (or lower width/minimum lateral extent of the intrusions.