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.