GIANT FLUIDIZED AND INJECTED SANDBODIES IN THE NORTH SEA BASIN- AN INDICATOR OF SHALLOW HYDROCARBON VENTING IN PALEOCENE-EOCENE TIMES?

Seismic-scale intruded sandbodies have been identified in association with deepwater depositional systems within Paleocene - Eocene successions in the Central & Northern North Sea Basin. A number of the intrusive sandbodies form part of important oil reservoirs in the area (e.g. Alba, Harding, Gryphon, Leadon and Balder fields). Core, log and seismic data from these oil fields reveals that (1) the sandbodies can have complex interconnected geometries on a millimeter to kilometer scale with dykes cross-cutting up to 200 m of stratigraphy and low-angle sill-like bodies with horizontal dimensions of up to 1000 m; (2) the parent source sands are pervasively fluidized and show few relic indicators of primary sedimentary depositional processes; (3) the dykes and sills emanate largely from the sides and the top of parent depositional sand bodies; (4) parts of the intrusion complexes are made up of fluidized, highly deformed sand-mud breccias and slurries which may have been extruded at the sea bed. These clastic intrusions are the largest ever documented within sedimentary successions, and much larger than the sand dykes, sills and volcanoes that form after liquefaction associated with large magnitude earthquakes. The extensive occurrence of the sand intrusions in deepwater successions of the Central and Northern North Sea implies that large volumes of fluid were migrating in the shallow subsurface in latest Paleocene - Eocene times. We hypothesize that the source of the fluid was hydrocarbons migrating upwards from the deeper mature kitchen areas in the basin. Oil, and probably gas, migrated into isolated sand bodies, weakly sealed by muddy sediments. The muddy seals failed and the overpressured gas- and oil-bearing sands fluidized and injected laterally and upward into the surrounding sediments.