ZORASTER, STEVEN, and STEPHEN BAYER, Landmark Graphics Corporation, Austin, TX

ABSTRACT: Three-Dimensional Fault Modeling from Cross-Sectional Data

A recent and significant improvement to modeling faulted horizon models for the petroleum industry has been the introduction and use of three-dimensional fault models. Three-dimensional fault models create opportunities for enhanced understanding of geologic environments.

Collecting fault data usually starts with the geoscientist interactively interpreting fault "profiles" from seismic sections, or from a geologic cross-sections program. Correlated fault profiles provide data for building 3-D fault models.

Many faults have relatively small dip angles. These faults can be modeled safely using industry-standard horizon modeling algorithms. However, modeling vertical faults, shallow listric faults, or "scissor faults" in the project (x, y, z) coordinate space often produces poor models. A solution to this problem is to model each fault in its own unique (u, v, w) coordinate system.

Defining a coordinate system for a fault involves calculating a "strike-dip" plane for the fault. The strike-dip plane is spanned by a vector in the project (x, y) plane which follows the fault strike, and by an orthogonal vector that follows the fault dip. The cross product of these vectors defines a vector and coordinate (w) normal to the fault plane.

A strike-dip plane coordinate system usually provides coordinates in which a fault surface, measured by the w coordinate, will be a single valued function of its (u, v) coordinates. The use of rotated fault coordinate systems leads to some computational problems; however, these problems are minor compared to the ease with which data to support modeling of faulted horizons can be extracted from 3-D fault models.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.