Fun with Dip! The Predictive Use of Dip Data to Improve the Interpretation of Fold Traps
Charles F. Kluth
Consultant, Denver, CO (formerly with Chevrontexaco, San Ramon, CA)
The orientation of planar geologic features (azimuth and amount of dip) are fundamental data in the analysis of structural traps. Inexpensive and easy-to-use tools have been developed to extract structural information from the change in dip within folds. These tools were originally developed and used in two dimensions. They are now being developed for use in three dimensions to make predictions about trap geometry, by leveraging data from where it is good and plentiful to areas where it is poor or sparse.
SCAT (Bengtson, 1982) is a tool developed for dip-meter data in wells, but can often be easily applied to surface data. SCAT analysis is made on a series of simple cross plots of components of the dip data (amount of dip, azimuth of dip, depth). The plotting can be done by hand, even in the field, or by computer, even at the well site. SCAT uses the fact that dip data are three dimensional, but until recent advances in analytical tools, were largely applied in two dimensions. The analysis is also inherently predictive because it highlights trends of changes in the dip data. Apparent dip profiles are a part of SCAT analysis that is particularly useful in surface data. We have used SCAT analysis to make quick and inexpensive improvements to structural interpretations around the world. We have recently used apparent dip profiles in the reinterpretation of parts of the Papua New Guinea fold belt as large detachment folds.
Isogons are lines in cross sections that link points of equal dip. The patterns of isogons in cross sections were used by Ramsay (1967) to classify folds. Isogon patterns can be used in a predictive way, however, when analyzing folds. The isogon patterns in cross section view and map view are both part of isogon surfaces that link points of equal dip in the entire volume of a fold that has curvature of the limbs. Therefore, dip data from the interpretation of key marker horizons can be used to constrain the geometry of the entire fold. The isogon surfaces have unique patterns for different types of structures, and for different styles of folds. The isogon patterns, therefore, have the possibility of being leveraged from areas where data are good and plentiful and projected into areas where they are poor or sparse. Faults, unconformities, and other points or surfaces where dips can change abruptly and unpredictably limit the utility and predictive power of isogon surfaces in some cases.
Another advantage of isogons is that they work in the time domain, in areas where the seismic velocities vary smoothly. In many areas then, advantage can be gained in understanding the structural geometry without converting seismic data to depth. The absolute amounts of dip are not the same in the time domain as in depth, but the patterns are similar and can be used in a predictive way. Isogons can often help interpreters decide which package of data is real and which is an artifact of the experiment, in seismic data with conflicting patterns of reflections.
The patterns of isogons are illustrated with a differentially inverted structure, and with data from Carter Creek field, a hanging-wall structural trap in the Wyoming thrust belt. The isogons in maps and cross sections of inversion structures show a distinctive v-shaped pattern that changes during inversion as the beds are rotated. We are working with the well-constrained structural geometry at Carter Creek to build tools to analyze isogons and traps in three dimensions.
Isogons are empirical, and contain no assumptions about ideal folding models. They represent the best way at present to analyze intermediate, non-ideal folds. Further, the deformation mechanisms of the folded layers are embedded in the geometry of the structure. It may be possible, therefore, to invert the isogon pattern to get information about the deformation mechanisms involved in the folding. The deformation mechanisms are often related to lithologies of the folded rocks, so it may also be possible in the future to make predictions of the sand/shale ratios in folds based on geometrical information embedded in the isogons. Finally, varying isogon patterns within a volume may provide a way of assessing the uncertainty of an interpretation, and lead to better assessments of trap risk.
AAPG Search and Discovery Article #90015©2002-2003 AAPG Distinguished Lectures