Mounded shallow gas sands from the Quaternary of the North Sea: analogues for the formation of sand mounds in deep water Tertiary sediments?

Abstract

Abstract Shallow gas-bearing sand mounds have been clearly defined by a recent three-dimensional seismic survey and are believed to result from large-scale remobilization of conventionally deposited sand. This concept has been invoked by recent workers to explain the formation of mounded sand geometries in Tertiary turbidite reservoirs of the Forth-Gryphon area northern North Sea. However, the idea has been slow to gain acceptance due to the unconventional mechanism and lack of analogues. In the Fisher Bank Basin area of the central North Sea unusual mounded gas anomalies have been recognized in the shallow subsurface at depths between 450 and 550 m subsea, most commonly occurring above deeply buried highs. Originally mapped as linear features on two-dimensional seismic data, three-dimensional seismic has revealed a circular to ovoid mounded geometry. The identification of fault zones extending up to this mounded horizon from deep in the section in an area where Upper Jurassic source rocks are gas mature may suggest a possible mechanism for mound formation. We propose here that gas has migrated to shallow levels along fault conduits into a conventionally deposited sand, and that this has resulted in sediment liquification. The movement of gas results in entrainment of sand grains towards subtle highs in the upper surface. Post-depositional processes thus result in the creation of a mound by withdrawal of sand from the flank areas and redeposition towards the centre of the feature.