Fault Seal Analysis In Carbonates

Fault Seal Analysis In Carbonates

There are some general points to note:
i) Ideally, you should be using a Vshale volume rather than a porosity volume to calculate fault seal as the software does not directly use porosity in the fault seal calculation.

ii) You can use the porosity volume together with seismic slices in order to show where on the fault surface you have high porosity units in the upthrown side that are in contact with high porosity units in the downthrown side.  This juxtaposition diagram will highlight potential leak points, from high porosity to high porosity, across the fault.  Likewise, you can also show high porosity against low porosity units

iii) A number of generalizations can be made about the fault zones in carbonates and their seal potential:

Fault zone fabric correlates with porosity of wall rocks, because the fault zone will tend to be a broad breccia fabric in highly porous carbonates and a narrow intense cataclastic fabric in low porosity rocks.  Narrow cataclastic rocks have higher seal potential than breccia fabrics so mapping the porosity distribution can be used to generalize fault rock type.   An alternative approach to mapping Gouge Ratio distribution on a fault plane is to map the contribution of wall rocks to the fault zone in terms of their porosity rather than their Vshale content.  This approach aims to correlate the contributed wall rock porosity with  fault rock type.  The lower porosity contributed to the fault zone, the higher the seal potential (=low porosity fault zone).

Fault throw is important in defining the seal potential of a fault because it influences the composition and nature of the fault zone.  At low throws (< 1m) the fault zone are relatively broad, comprising breccia zones en echelon vein and fracture arrays with overall high zonal permeability.  At moderate throws (1-10m) fault zones comprise a high permeability breccia zone with high density fracture damage zone.  At large throws (>10m) a wide low permeability cataclastic / breccia zone is developed together with a broad zone of high density fracture damage.  Taken together increasing seal potential correlates with increasing throw.

Crucially, fault zone permeability is strongly affected by fault geometry.  Variation in the make up of the fault rock at complex fault intersections can lead to local high permeabilities and low seal potentials.  Increased brecciation and fracturing associated with the linkage of fault segments, along zones elongate in the fault slip direction leads to higher permeabilities.  The evolution of segmented fault arrays within carbonate rocks, provides zones of enhanced permeability at pre-existing segment boundaries.

Fault zone permeability is also affected by the materials contributing to the formation of fault zone through smear and injection processes.  It is well known that contribution of clay or shale to the fault zone can increase the seal potential of a fault zone, mobile evaporites such as halite and anhydrite can also contribute to occlusion of pores through smearing of chemical re-precipitation.

iv) All the above information and attached files outline the development of seal developed along faults in carbonate sequences.  Fractures developed within the carbonate reservoir itself may exert a first order control on the accumulation and/or loss of hydrocarbons from a carbonate reservoir.  You should also consider the analysis and prediction of the fractures that may occur within the reservoir itself.

v) Finally note that there is a paucity of information regarding sealing behaviour in Carbonate lithologies, and as a result it has not been possible for us to accurately predict their behaviour to date.