Pete Bretan¹, Graham Yielding¹

(1) Badley Technology Ltd, Lincolnshire, United Kingdom

ABSTRACT: Routine Testing of Fault Traps

'Best practice' in fault seal analysis is dependent on the available data. High-quality input data are essential, in particular good fault mapping in three dimensions. A first step is to build a geologically-realistic framework model using fault and horizon picks from seismic interpretation or mapping. Additional layers must be incorporated into the model because seismic interpretations on their own rarely contain sufficient lithostratigraphic detail. The full framework model provides 3D visualisation of reservoir and seal layers at the fault surface and permits routine testing of traps in three dimensions.

Once reservoir juxtaposition relationships are established, the routine assessment of the nature of the fault-zone material is necessary to predict the likely behaviour of sand-on-sand contacts. One such predictor is the Shale Gouge Ratio, or SGR, an estimate of the proportion of shaly material in the fault zone. The basic assumption in the SGR algorithm is that the fault-gouge composition is governed by the bulk composition (Vshale) of the wall rocks that have slipped past that point on the fault.

A critical step in applying SGR is to calibrate it against known subsurface fault behaviour. This involves mapping in-situ pressure differences onto faults to identify and characterise the unbreached parts of sealed faults. Compilation of many SGR analyses with in situ pore-pressure data has allowed a better definition of the relationship between calculated SGR and maximum trapped hydrocarbon column height i.e. the 'fault-seal failure envelope'. A family of such envelopes can be defined for targets with differing geological histories.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado