Influence of syn-depositional faulting on thickness variations in chalk reservoirs – Valhall and Hod fields

Abstract

The depositional model for the Tor Formation at the Valhall and Hod fields has been revised to reflect the influence of syn-depositional faulting and reworking on reservoir thickness variations. New ideas for the structural and depositional model were based on detailed reservoir interval correlations in the Valhall–Hod area using biostratigraphy and graphic correlation techniques. Three-dimensional seismic data was also integrated and applied to develop the model. The Tor Formation was deposited during a tectonically active time in the Late Cretaceous and the Early Tertiary. During this time Valhall and Hod anticlines were forming as a result of inversion along the Lindesnes Ridge. Regionally the Tor and Hod formations thicken away from the Ridge. This indicates that the area was a structural high which is interpreted to be a large shoal. A major unconformity is present at the Hod/Tor Formation boundary. During continued uplift, crestal areas collapsed forming a series of horsts and grabens. The Tor Formation (Campanian and Maastrichtian, with documented Danian reworking) filled in the lows formed by these graben. Seismic data and well control demonstrate that the Tor Formation is thicker in graben and thinner over horsts. Removal of the Hod and Tor formations from local structural highs can be best explained by wave action and winnowing across a large shoal. The graben areas, sheltered from waves and protected from scouring and erosion, became local depositional centres for thicker slumped and reworked Tor Formation chalk. In the Tor Formation there is strong evidence for local erosion, formation of hard-grounds, grain size sorting, and down-slope movement through slumping and debris flows. Thickness and facies variations in the Tor Formation of Valhall and Hod fields can be linked to syn-depositional faults. Recognition of a fault controlled depositional model along with detailed mapping of thick reworked chalk in local graben led to the definition of additional reserves in the Valhall–Hod area.