Abstract: Statistical Curvature Analysis Techniques for Structural Interpretation of Dipmeter Data

C. A. Bengtson

Although the variety of possible subsurface structural shapes is seemingly unlimited, the curvature of these shapes is governed by natural constraints that limit the ways that dip can vary in the subsurface. Because of these constraints, only a limited number of valid statistical patterns are seen on graph-type displays of dipmeter data. In particular, plots showing angle of dip versus azimuth of dip for either an entire well, or parts of a well above and below angular unconformities or certain kinds of faults, will show only one of six possible statistical patterns, each indicative of a specific structural setting. From each pattern (except for zero dip) mutually perpendicular directions of least and greatest structural change (the L- and T-directions) can be determined. /P>

The L-direction is parallel with the strike of the beds in planar-bed settings and parallel with the "strike" of folds and faults in curved-bed settings. L-direction dip component versus depth plots accordingly show simple patterns corresponding to nearly uniform apparent dip. T-direction dip component versus depth plots, in contrast, show simple to highly complex patterns related to the curvature of the beds as seen on transverse cross sections. Crestal planes, axial planes, inflection planes, and dip-slip faults are located and identified by "special points" on the statistical trend line. Azimuth of dip versus depth plots show relatively simple patterns related to the curvature of the beds as seen on structure contour maps, but angle of dip versus depth plots cannot be interpreted o a geometrical-statistical basis except in simple situations.

By fitting internally consistent statistical trend lines to the previously described data plots, structurally representative dips (i.e., dips that would be derived from correctly drawn maps and cross sections) are established along the well bore. These dips can be extrapolated on transverse cross sections using auxiliary construction lines known as dip isogons, thus generating dipmeter-controlled sections from which structure contour maps can be derived, by projecting the data parallel with the average L-direction dip component.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma