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BOYER, STEVEN E., Department of Geological Sciences, University of Washington, Seattle, WA; and JAMES D. MORSE, Computational Geology, Boulder, CO

Analysis of Thrust-Related Fold Geometries Utilizing SCAT and Tangent Plots: Implications for Well Prognoses and Reservoir Calculations

SCAT and tangent plots of real folds and models indicate that fault- propagation folds change shape along strike from cylindrical to conical. The opposed conical terminations of a fault-propagation fold broaden down-plunge in the direction of lateral fault propagation and are joined at their up-plunge apices by a cylindrical culmination. Two opposed kink bands, joined by an undeformed interlimb region, define each lateral termination. Data from either kink band and the adjacent interlimb band plot as a cylindrical fold, but data from both limbs and the interlimb region describe a non circular cone generated by the up-plunge convergence of the two cylindrical fold segments.

Fault-propagation folds also change with depth. The opposed kink axial planes intersect at depth, defining a chevron core. In the cylindrical culmination, the line of kink-plane intersection lies in bedding, and the trend and plunge of the structure remain constant with depth. However, in the plunging terminations of the fold, the kink-plane intersection does not lie in bedding and plunges at a higher angle than does the axis of the chevron fold. As a consequence, a broad box-shaped structure in the near surface may plunge at a steeper angle than does the tight, chevron fold at depth.

Interpretation of such complex structures is aided by SCAT and tangent analysis, methods that are especially useful and cost-effective when applied to real-time analysis of exploratory wells, prediction of closure, and estimation of reserves.

AAPG Bulletin, v. 78 (1994), n. 13 (Annual Meeting), p. 109

James D. Morse is one of the founders of Computational Geology.

Steven E. Boyer is an instructor on the AAPG Field Seminar, "Fractures, Folds, and Faults in Thrusted Terrains: Sawtooth Range, Montana". Dr. Boyer also teaches a Public Course, "Compressional and Transpressional Structural Styles" for PetroSkills.