Fault seal analysis: successful methodologies, application and future directions

Fault seal prediction in hydrocarbon reservoirs requires an understanding of fault seal mechanisms, fault rock petrophysical properties, the spatial distribution of seals, and seal stability. The properties and evolution of seals within fault zones can be evaluated using the combined results of structural core logging, microstructural and physical property characterisation, together with information on fault populations from seismic and outcrop studies and well test data. The important structural elements of fault zones which require characterisation are: u — the microstructural/petrophysical properties of the different fault rocks present; — the population of faults and fractures which define damage zones around large faults; — the spatial distribution, orientation and clustering of the deformation in individual fault zones; — the history of fault activity, diagenesis and migration; — the distribution and volume of fault rocks with different properties. Fault rocks in siliclastic sequences range from quartz-rich cataclasites, developed from pure sandstones, to phyllosilicate smears developed from shales. Fault rocks developed along sand/sand fault juxtapositions can have transmissibility reduction factors of >10 6 . The exact value depends upon the conditions of faulting and the amount of self-sealing experienced by the fault rock. An important class of intermediate fault rocks are those generated from impure sandstones, or from sandstones with concentrations of fine phyllosilicate laminations. The localisation of cement precipitation within the damage zone may occur, which will remove the applicability of simple seal prediction based only on the host-rock lithology and fault displacement. The density of structures present in damage zones around faults is related to the cumulative displacement across the zone. The detailed internal structure of a fault zone is dependent on the conditions of deformation, the lithological architecture present and the position in the fault array. Successful seal analyses depends upon the amalgamation of data from the micro-scale to the macro-scale. This review demonstrates that improvements in fault seal risk evaluation are possible. The future directions for improving fault seal risk evaluation are also discussed. The most critical of these are; characterisation of the internal structure of fault zones, generation of a database for fault rock petrophysical properties and incorporation of the impact of realistic fault zone geometries into reservoir modelling programs.