SANDSTONE REMOBILISATION IN THE PALEOGENE DEEP-WATER SANDSTONE RESERVOIRS OF THE NORTH SEA: EXAMPLES FROM THE ALBA FIELD AND GRYPHON FIELD

Numerous Paleogene deep-water clastic reservoirs of the North Sea display unusual geometries and facies. The Alba Field (Upper Eocene) and the Gryphon Field (Upper Paleocene and Lower Eocene) are two of these relatively small and isolated sand accumulations. Cores from these fields reveal a large amount of sandstone dykes and sills and hydraulic breccias, which can make up tens of feet-thick injection complexes. These units commonly overlie hundreds of feet-thick sandstone bodies encased by shales. The thick bodies are mainly composed by massive sandstone, with sparse consolidation laminae and dish structures. Current lamination or other depositional structures are very rare in these units. On the contrary, evidences for large-scale pore fluid escape occured during early burial are numerous. Metre-scale pillars, sequences of oversteepened laminae and thick sand units pervasively disturbed by small dewatering structures indicate that wholesale fluidisation affected the sandbodies. Core sequences reveal that this process led to disruption and homogenisation of originally laminated sandstones, brecciation of the overlying shale and sand intrusion. Injected units frequently include several granulation seams, whose origin is likely to be related to post-emplacement compaction. Thin sections and SEM analysis indicate a higher amount of clay and quartz-overgrowth in the injected sands. Mounded geometries with steep flanks, wing-like features and ridges cross-cutting the stratigraphy characterise their seismic-scale geometry. Their limited extension and efficient shale sealing, determined the overpressure build-up, which was probably favoured by a gas charge. Gas vent structures have been recognised in the core record. Sandstone remobilisation greatly modified geometries and facies distribution of these turbidite reservoirs, hampering the application of the most common facies models. Large-scale sand remobilisation is inreasingly being reported from various deep-water settings, revealing this process as important controlling factor of turbidite sandstones organization.