Modeling the Difficult Faults

Karen Hoffman, John W. Neave, and Erik H. Nilsen
Roxar, Inc., Houston, TX

Many of the commonly used fault modeling techniques can adequately handle simple structures - even when those structures contain large numbers of faults. It is not the sheer number of faults that is often the stumbling block, but rather the complexity of the fault/fault relationships. Common configurations such as a Y intersection formed by a synthetic/antithetic fault pair can be difficult to model due to the limitations of the framework-building technique. Often the truly difficult situations, such as listric faults, low angle faults, or crossing conjugate faults, are not even attempted - which means that the resulting reservoir models do not correctly represent the subsurface. Our technique for building a fault framework provides a comprehensive solution for modeling any type of fault shape or fault intersection. Listric faults and Y-intersections, for example, must be treated as special cases with pillar-based methods, and a nested-Y structure is nearly impossible to model with this technique. Low angle faults and low angle intersections also pose problems for pillar-based methods; the low angle intersection can distort the shape of one or both faults. Crossing conjugate faults can create half-Y faults, which neither the pillar method nor the binary tree method can model. The concept of compound relationships used in our approach, where faults are divided into sections based on their relationships with all faults which touch or cross it, allows independent manipulation of each small section of a fault. In this way, even the complex truncations of crossing conjugate faults are easily handled. The solid models and reservoir grids that are created based on these frameworks incorporate all of the complexity of the interpretation. These models provide a better basis for reservoir management decisions and can be shared by all of the disciplines within an asset team.