Abstract

The Medina anticline is a fault-bend fold located in the hanging wall of the Aguaclara fault along the southeastern thrust front of the Eastern Cordillera of Colombia, a petroleum province with giant fields. We employ a modified technique of axial-surface analysis using fold-evolution matrices, which helps define the subsurface geometry and thereby provides important constraints on trap geometry, modeling of hydrocarbon migration pathways, and prediction of reservoir quality.

Fold-evolution matrices illustrate the effects of two independent variables on cross-sectional fold geometry. Three matrices are presented for fault-bend folds in which the fault shape is controlled by a preexisting rift-basin architecture. In each case, variation in displacement is plotted against change in fault geometry: increasing ramp height, decreasing ramp dip, and increasing length of an intermediate flat. The matrices are used to construct synthetic axial-trace maps and corresponding block diagrams in which one or both parameters vary along strike. The synthetic maps can be compared to axial-trace maps of real folds and used in conjunction with other data and methods to provide a better understanding of fold geometry and genesis.

Axial-surface analysis of the Medina anticline shows that the three-dimensional fold geometry is compatible with a model in which three parameters vary along strike: (1) increasing fault displacement to the northeast, (2) increasing length of an intermediate flat to the northeast, and (3) decreasing ramp dip to the southwest. The model is used to construct the three-dimensional fault geometry, which is characterized by a ramp-flat-ramp segment to the northeast and an oblique ramp to the southwest. The Medina anticline formed during Tertiary inversion of a Jurassic rift basin, where preexisting normal faults controlled the position and orientation of ramps in the Aguaclara fault.

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