Fault Zone Heterogeneity, Scaling, and Application to Exploration and Reservoir Modelling

Abstract

The widths of architectural elements of fault zones can strongly influence reservoir behavior, but are often too small to be directly measured on seismic data. For example, fault core (FC) width is needed to predict cross-fault flow rates, but most FCs are on the order of meters wide. A common predictive approach assumes that FC width increases with fault displacement. A compilation of 1800 measurements shows that this relationship exists for displacements < 100-300 m, but that once this value is reached, FC width plateaus. The recognition of a threshold value is important because if a single, monotonously increasing relationship is used then FC widths for large faults will be over-predicted. This means that the transmissibility of these faults would be under-estimated. There is also an indication that displacement can be related to fault rock type in some lithologies. For example, below displacements of ≈200 m, faults in carbonates are dominated by permeability-enhancing breccias. Above that value, faults with permeability-reducing cataclasite become much more common.

The compilation can also be used to calculate probability distributions for FC widths. For example, ≈30% of reservoir-scale faults (disp. of 10-100 m) have FCs widths exceeding 1 m, compared to ≈80% of trap-scale faults (disp. > 100 m). Subseismic faults (disp. of 1-10 m), which may be important in structurally complex fields, have only a 3% chance of containing a FC wider than 1 m. These FC widths can be used to calculate fault transmissibilities, but may also have implications for risking fault rock continuity, with thinner FCs being more likely discontinuous at a trap scale.

A compilation of 2900 measurements shows the existence of a threshold value at a displacement of ≈300 m for total fault zone width. Previous studies have proposed significantly different thresholds at displacements ranging from 1 to 2400 m. The accurate definition of a threshold value is important as it can be used to constrain the distribution of fault zones/damage zones in faulted reservoirs. These zones can either enhance or reduce reservoir permeability depending on whether they contain permeability-enhancing structures such as fractures or permeability-reducing features such as deformation bands. In a realistic geological model, fault zone widths should strongly scale with displacement for displacements below the threshold value. Above that value the rate at which fault zone width increases should be reduced.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018