Abstract: Geometry and Evolution of Structural Traps Formed by Inversion Structures

Shankar Mitra

Inversion structures form by compressional reactivation of preexisting extensional structures. Experimental models and observations of natural structures are used to develop quantitative

models for the geometry and kinematic evolution of inversion structures. Two main mechanisms of formation of inversion structures are analyzed: (1) fault-propagation folding on planar faults, and (2) fault-bend folding on listric faults. Inversion structures formed by fault-propagation folding are characterized by the upward termination of a basement fault into a tight fold and thickening of synextensional units into the basin. Inversion structures formed by fault-bend folding are characterized by open-fold geometries and thickening of synextensional units into the fault zone. Characteristic variations in fold geometry and bed thickness provide predictive models for interpreting the subsurface geometries of these two classes of inversion structures in areas with poor seismic data. Exa ples of both types of structures are described from the Taranaki basin, the southern North Sea, and the Kangean Basin.

The structural-stratigraphic evolution of inversion structures can result in ideal conditions for the trapping of hydrocarbons. Some critical factors that may determine the hydrocarbon potential of inversion structures are the maximum depth to which the source rocks were buried during extensional deformation, the timing of generation and migration of hydrocarbons relative to the timing of extensional and compressional deformation, and the preservation of a structural trap and seal during inversion. Because of their complex structural history, kinematic restoration of inversion structures through their structural history is essential for evaluating their hydrocarbon potential.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994