HOFFMAN, KAREN, and JOHN NEAVE , Dynamic Graphics, Incorporated, Alameda, CA
Understanding the sealing characteristics of faults is critical in assessing the hydrocarbon potential of traps formed by faults. Fault gauge ratio and juxtaposition analysis have often been limited to a single cross section (a two-dimensional approach) or to a single, isolated fault surface (a partial three-dimensional approach). A full three-dimensional solution has now been developed that calculates fault gauge ratio using a continually varying clay volume fraction, a network of faults (isolated, dying, and/or branching), and displacement along the fault surface (instead of just the dip component). Geometric reconstruction model-building techniques construct faults and horizons in three-dimensional space, allowing easy and rigorous calculation of juxtaposition and displacement.
The steps in calculating fault gauge ratio in three-dimensional space are as follows: 1) create structural framework; 2) calculate Vcl as continually varying property within this structural framework; 3) determine displacement, allowing for oblique slip; and 4) calculate fault gauge ratio for both hanging wall and foot wall blocks, and sum to determine the ratio for every point on the fault.
Rigorous calculation of fault gauge ratio depends on a robust structural model. With this model, a variety of scenarios may be investigated, thus incorporating uncertainty into the calculation. Determining whether a fault will act as a seal, or whether there is potential for development of leaks during the production of the reservoir depends on many variables. Minimizing the uncertainty of this part of the analysis may provide greater confidence in assessing the risk.
AAPG Search and Discovery Article #90908©2000 GCAGS, Houston, Texas