Sand injectites in the Northern North Sea: Mapping and understanding sand injectites on the Fram Field

Abstract

Sand injectites are found in several stratigraphic positions in the Cenozoic succession, over large parts of the North Sea. Sand injection occurs when the fluid pressure in a source sand exceed the lithostatic pressure, and the overlying sealing lithology is hydrofractured. The source sand is fluidized and transported up through the fractures, resulting in sand injection upwards in the stratigraphy. If the injected sand reaches the free surface it is termed a sand extrudites.

14 wells and 500 km2 of 3D seismic data covering the Fram Field area in the northern North Sea has been interpreted to map the occurrence of sand injectites and extrudites. 31.2 km3 of sand injectites or extrudites have been observed in the Cenozoic strata in the study area, in two separate stratigraphic complexes. The first injection complex is located within the upper Rogaland Group and an estimated volume of 5.8 km3 of sand is present as conical or saucer shaped sand injectites. No extruded sand was identified. The second sand injection complex is located within the upper Hordaland Group and caused large scale mounding of the Top Hordaland Group surface. An estimated minimum sand volume of 25.5 km3, 8.8 km3 in conical shaped sand injectites within the Hordaland Group strata and 16.7 km3 of extrusive sand present above the top Hordaland Group unconformity, was re-mobilized during the injection phase.

Both injection phases have been observed to utilize existing weaknesses such as polygonal faults and erosional structures, when they formed. The first injection phase coincided with the opening of the North Atlantic. The associated tectonic stresses may have acted as an external triggering event for injection. No tectonic event is found to coincide with the second injection phase. External triggering events are therefore discarded as a necessary prerequisite for sand injection in the study area.

The source sand interval for the second injection phase is suggested in the lower Hordaland Group, because isopachs suggest partial depletion of the section. For the Rogaland Group sand injection complex no source sand interval has been identified, but a Paleocene source sand is considered most likely. Injection occurred within 200 m of the paleo-surface for both sand injection phases, a probable maximum burial depth for the source sand before sand re-mobilization in the Fram field area is found to be 900 m.