oa Top Seal Assessment in High Pressure – High Temperature Plays – Evidence from the UK Central Graben

The current methodology of predicting top seal integrity in high-pressure/high-temperature (HPHT) plays (Gaarenstoom, et al., 1993) is to assume that the caprock, defined as the low matrix permeability formation immediately overlying the reservoir, is the seal (e.g. the Kimmeridge Clay Formation above older sandstone of the UK Central Graben Mesozoic play fairways). This study challenges this assumption and proposes the existence within the caprock of a fluid waste zone consisting of a system of fractures cutting from the reservoir up into or even through the caprock and therefore charged with reservoir fluids. Because of the waste zone the caprock cannot seal the underlying reservoir fluids. Seismic and elastic logs data help identify any fracture waste zone and also any stratigraphic boundary capable of containing it (or arresting its upward propagation).