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

C. A. Bengtson

Statistical curvature analysis provides guidelines for interpolation and extrapolation of dip data that make it possible to draw maps and cross sections for limited regions around a well using only the dipmeter data for the well. Five computer-derived displays are required: (1) angle of dip versus azimuth of dip, independent of depth; (2) L-direction dip component versus depth; (3) T-direction dip component versus depth; (4) azimuth of dip versus depth; and (5) angle of dip versus depth.

Dip versus azimuth plots for any well (or parts of a well above or below an angular unconformity or certain kinds of faults) show one of five 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.

The L-direction parallels the strike in homoclinal dip settings and parallels the structural b-axis in fold and fault settings. L-direction dip component plots accordingly show simple patterns corresponding to nearly uniform apparent dip. T-direction dip-component plots, in contrast, show nine possible patterns (not counting discontinuities) that occur singly or in combination. Each pattern identifies a particular kind of bedding geometry or a specific kind of dip-slip fault. Azimuth versus depth plots show six patterns consistent with the L- and T-direction plots.

Cross-section construction begins by fitting internally consistent statistical trend lines to all five data displays. Dip bars derived from the T-direction trend line are entered at convenient depths along the well. Crestal planes, axial planes, inflection planes, and faults, if present, are identified by "special points" on the T-direction trend line and entered at their appropriate depths. A freehand section now can be drawn, but better results are possible using auxiliary construction lines known as dip isogons. When the cross section is finished, structural elevations and analytically derived dip-strike symbols for any chosen horizon are projected perpendicular to the cross section in both directions, using the L-direction dip component to raise or lower the elevations. This proce ure establishes a network of control for a contour map centered at the well.

AAPG Search and Discovery Article #90963©1978 AAPG/SEG/SEPM Pacific Section Meeting, Sacramento, California