Fault Seal and Weak Points; a Case Study from Njord

Erling Rykkelid
Norsk Hydro, Oslo, Norway

Position and properties of weak points controls the sealing potential of a fault. Average properties of a fault are less relevant, and properties deducted from lithology in wells may strongly overestimate seal due to the heterogeneous nature of a fault rock. A well through a fault is not likely to hit the local weak point.

Detailed mapping of fault geometry and displacement gradients may help the identification of weak points on faults. A case study from the producing Njord field, Norway, shows faults interacting in 3D and with a characteristic wall rock deformation assisting the interpretation of fault seal. The Njord Field is suited for these studies because reservoirs are dissected by a number of faults, the seismic data quality is decent, and pressure control from a number of wells make it possible to link fault seal variations to the different geometric settings. The reservoirs are two Jurassic shallow marine/fluvial sands (Ile and Tilje Fm) separated by a sealing flooding surface (Ror Fm). These two reservoir layers have different OWC's and GOC's. Single faults do not have the same sealing property in the two reservoir zones.

One example is a fault that is partly sealing for the upper reservoir layer, and that has mainly contractional overlap. This overlap is associated with a slight reverse drag and higher offset. In a smaller area, fault interference with extensional overlap results in local normal drag and less offset, and this seems to control leakage and the present OWC in the upper reservoir layer.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005