Cross-cutting sand bodies of the Tertiary, Beryl Embayment, North Sea

The Lower Tertiary Balder Formation in the Beryl Embayment, North Sea, consists of sands interbedded with claystones and tuffs. The sands are massive and well sorted and can be up to 400 feet (122 m) thick, and are highly porous and permeable hydrocarbon reservoirs. The sands form large lobate and circular bodies of sands a few kilometres in diameter, with steep sides that are sometimes controlled by fault planes. The margins of the sands sometimes display thick sand wings extending up along fault planes. The sands display dewatering structures such as sills and dykes have a complex geometrical relationship with the surrounding sediments. Hydrothermal mineralisation is displayed as nodules, concretions and cementation of the sands within the Balder interval. The concept of seismic pumping, which postulates the rapid upward migration of deep fluids as the result of fault movement, was introduced to the literature some eighteen years ago, but fell into disrepute. However, it is argued here that re-shear of normal faults in the reverse direction can under certain critical physical conditions cause seismic pumping and can transport large quantities of deep seated fluids rapidly. This gives rise to the expulsion of fluid from depth into conventionally deposited massive sands of submarine fan environments, belonging to the Balder Formation, and thus in the fluidisation of the sediments. These sands have been intruded into the surrounding rocks and along fault planes forming a complex distribution of in situ and remobilised sands, thereby giving rise to the observed sand geometries and structures