Abstract

The current model describing the sealing mechanism of high-pressure hydrocarbon traps and the ensuing methodology for predicting top seal integrity and capacity in high-pressure plays assumes that the caprock, defined as the low matrix permeability formation immediately overlying the reservoir, is the seal.

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 upwards into the caprock and, therefore, charged with reservoir fluids. Because of the waste zone fractures the reservoir fluids are not sealed at the base of the caprock. Instead the seal coincides with the fracture waste zone tip point, which occurs at an important stress and stratigraphic boundary termed the ultimate seal. Six case studies demonstrate that in the UK Central Graben the top seal for Mesozoic high pressure hydrocarbon accumulations lies between the Base Cretaceous Unconformity and the base of the Chalk Group and that the Jurassic Kimmeridge Clay Formation is not necessarily the seal for these traps. The data used for the construction of structural and stress models for the case studies include pore pressure measurements, formation integrity measurements, well logs and reflection seismic profiles.

An important conclusion of this study is that the shorter the waste zone the higher the chance of finding a hydrocarbon column in the reservoir.

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