Fault network modeling of complexly faulted structures - those containing hundreds or thousands of faults - can be an extremely difficult and time-consuming process. A variety of methods exist to create the appropriate relationships between the faults, due to truncations, crossing, or offsets due to younger episodes of faulting, but each has limitations. Pillar or node-based techniques are often based on the concept that faults only exist where there are interpreted data; extensions of faults to intersections with other faults and/or truncation of erroneously crossing faults is a manual process, requiring the addition of nodes or pillars to the truncating surface. The location of the intersections can be dictated by the orientation of the pillars of the two faults, and so may not reflect the correct structure. An automatically generated binary tree, or hierarchy, can generate a reproducible fault network, where fault relationships can be stored. However, a binary tree cannot correctly handle all types of fault relationships and has particular difficulty in easily representing crossing faults.
Our technique uses a new concept of "fused" fault blocks. It begins with an approach somewhat similar to a binary tree, where each fault defines a hanging wall and foot wall side. However, our technique does not require faults that have an implicit relationship (that is, faults which do not intersect) to be explicitly defined in a tree. Fault intersections are determined from the active areas of the faults. Unlike a strict binary tree, fault relationships here can be modified at any time, as interpretation or data changes.
Truncations can be stored and re-used, making the fault network repeatable. The accuracy of the fault network, the flexibility of modeling, and the use of this network to create reservoir grids allows an entire asset team to work with a high-quality earth model.
The vast majority of oil and gas fields around the world are faulted, and it is necessary to incorporate faults into mapping and modeling in order to properly calculate reserves and plan wells. Although techniques for 3D modeling have been in existence for a number of years, many fields are still modeled using 2D map techniques. This may be partly due to the fact that it can be difficult to correctly represent the true fault framework in many of these fields due to restrictions of the modeling techniques or the difficulty in using the available software applications. The benefits of working in 3D may not outweigh the time and effort needed to create the model.
The technique that we have developed for fault modeling overcomes many of the obstacles, in both the solution to difficult fault networks and the ease of use of the software application.
The fault network modeling methods available today can be generally divided into two categories: pillar or node-based methods and binary tree methods. Each of these has its advantages and disadvantages, but neither method can handle the full range of fault intersection types.
In the pillar, or node-based, method, the fault network is created by drawing lines which represent the trace of a fault (on a horizon, at a constant depth, or at any position), intersecting those lines at nodes, and then extending pillars from the fault lines. These pillars are used for several purposes, including control of the 3D shape of fault/fault intersections and adding control to the fault surface itself.
The advantages of the pillar method are:
it is interactive;
faults can easily be truncated against one another by extending fault lines; and
the geoscientist can easily add soft knowledge to the fault network without having to create data.
For simple models, with perhaps tens of faults and no complex intersections, this method is quick and easy.
The drawbacks to this approach, however, are several:
faults are present throughout the entire stratigraphic section;
pillars at fault intersections limit the type of intersections that can be modeled;
Y-faults are a special case; and
fault relationships cannot change along the length or depth of the fault.
These disadvantages are briefly described below.