It is imperative that anyone involved in the exploration for or exploitation of hydrocarbons have a significant understanding of faults in the area of study, their origin and relationship to the forming of structures. Detailed interpretation and mapping of major faults is critical in the process of hydrocarbon exploration and development. Faults themselves are vital to structural development, hydrocarbon migration and entrapment. The interpretation and mapping of faults is not an academic exercise. They are required to accurately define a structure, the associated reservoirs and the ultimate determination of accurate reserves. A reasonable structural interpretation, in faulted areas, begins with an accurate fault interpretation resulting from the construction of fault surface maps and the proper integration of these fault maps into the structural interpretation.

The preparation of accurate fault surface interpretations and maps requires a strong geological background, three-dimensional thinking, a good understanding of the structural style of the area being worked and an accurate method for contouring the fault. When we make reference to the understanding of structural style, we are referring to that specific assemblage of geologic structures common to a particular petroleum province. In order to prepare geologically reasonable maps, one must be familiar with the tectonic setting being worked, the fault and structural patterns expected, their origins and at times the process of development. Many of the basic concepts, methods and techniques for interpreting and mapping faults are universally valid. However, their recognition, interpretation, map construction and application very much depend upon the geoscientist's background and understanding of the kinds of geologic structures being worked.

The construction of fault surface maps has numerous benefits in the interpretation and understanding of the development of faulted structures. Fault surface maps:

1. aid in solving three-dimensional structural problems;
2. define the location of a fault in space in both the horizontal and vertical dimensions;
3. help delineate complex fault patterns;
4. can be integrated with structure maps to delineate accurately the:
   1. upthrown (footwall) and downthrown (hanging wall) fault traces
   2. fault gap or overlap
   3. hydrocarbon reservoir limits;
5. are required to evaluate potential cross-fault drainage;
6. are required to determine the correct reserves for a faulted reservoir;
7. can be used, at times, as an indication of the changing stratigraphy (sand/shale) in the footwall by means of a change in dip of the fault;
8. are used to determine potential reserves in a fault wedge;
9. can be used to estimate the dip and strike of a fault at any location along the fault;
10. aid in the designing of well plans, particularly for directionally drilled wells; and
11. help evaluate the validity of a prospect that is fault dependent.

In today's world of computer generated maps, an interpreter must be able to define the method by which a fault and other surfaces are to be contoured. This means not only selecting the appropriate algorithm but ensuring that the final map makes geologic sense in three dimensions.

Figure 1. is a fault surface map on a relatively simple fault. The data came from 18 vertical wells. The fault is interpreted as a listric syndepositional growth normal fault with a curvilinear concave - upward surface; that is a fault whose dip decreases with depth and the missing section increases with depth.

FIGURE 1

The Projected Slope gridding algorithm was used for Fig. 2. The result is a map similar to the hand-drawn map in Fig. 1 in that the fault surface is listric but has a smoother curving lateral bend. A Least Squares gridding algorithm generated a comparable map except that the extrapolated 5000-ft contour did not conform well to the trend of the other contours and the surface was not so smoothly listric. A Closest Point algorithm was used to generate the fault surface map in Fig. 3, results in a fault surface that is not geologically reasonable considering the data, which indicate that the fault is a large growth fault. The mapped surface is not listric and the contours are not credible at the limits of the data, both shallow and deep. The Closest Point algorithm is best suited to a data set with more numerous and more evenly distributed data points than in our example. Lastly, Fig. 4 depicts a bogus fault surface map generated by an algorithm (Point Density) that is unsuitable to the data set. These three examples demonstrate how critical it is to select an appropriate gridding algorithm (including suitable gridding parameters) for generation of a fault surface interpretation or for integration with a structural horizon map. In addition, it is important to note that most computer generated maps require hand editing or correcting in order to best represent a geologic surface.

FIGURE 2

FIGURE 3

FIGURE 4

Comparison to the hand-contoured map in Fig. 1 indicates that the Projected Slope algorithm was the best choice among four algorithms to generate the most reasonable fault surface interpretation. But that does not mean the Projected Slope algorithm is always the most suitable for mapping a listric fault surface with a lateral bend. The best algorithm is dependent on the number and distribution of data points, among other things. Beware of habitually choosing the same gridding algorithm in computer-based mapping. The interpreter must be sufficiently familiar with the various algorithms and gridding parameters in order to choose the one most suitable to the data set and the geologic surfaces in the area of study.

In developing a final fault surface map interpretation, keep in mind that a fault need not remain constant in strike direction, dip, or displacement over its entire extent. Along strike, the displacement may increase, decrease, or remain constant, and the strike direction may change. The displacement might increase with depth, decrease to zero upsection, or even decrease with depth. A fault may die laterally, have its displacement transferred to other faults or to folds, combine with other faults, or intersect with or terminate against another fault. We again emphasize that a good interpretation of a fault must have three-dimensional validity and comply with the tectonic characteristics of the region being mapped, and you must use the correct mapping techniques in its construction.

For more detailed information on faults, their importance, interpretation and mapping refer to Tearpock and Bischke 2003 "Applied Subsurface Geological Mapping with Structural Methods", Edition 2, published by Prentice Hall, at www.scacompanies.com.

Copyright© 2003, Subsurface Consultants & Associates, LLC