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Fault History Diagrams: Rapid Analysis of Temporal Variations in Fault Throw and Lithological Juxtaposition

Abstract

Two-dimensional fault strike-projections allow rapid visualization of fault throw or across-fault lithological juxtaposition (Allan diagrams). Strike-projections are constructed by projecting hanging wall and footwall cut-off lines onto a vertical plane oriented parallel to the average strike of the fault surface. Throw is calculated from the vertical separation of the cut-off lines and interpolated across the diagram. Lithological juxtapositions of hanging wall and footwall stratigraphies, for example sand/sand overlap, are derived from the across-fault relationships of the cut-off lines. More detailed lithological variations, and thus juxtapositions, can be obtained by projecting Vshale values onto the diagram from proximal well data. Structural restoration techniques are incorporated into strike-projections to construct 2D “fault history” diagrams, which allow the temporal development of throw and juxtapositions to be quickly analysed. At each horizon time-step, the effects of physical compaction are restored using industry-standard compaction curves, and throw is restored to the footwall cut-off line. Whilst fault history diagrams represent an approximation of a 3D surface, the method provides initial insight into the temporal change in fault properties ahead of a full 3D restoration. The diagrams can, therefore, save time and costs associated with further, more in-depth analyses. In this work, fault history diagrams are used to analyse the development of the Cape Egmont Fault, located in the Taranaki Basin, offshore New Zealand. The structure, a regional scale normal fault, is a proposed flow pathway for the charge of the adjacent Maui gas condensate field. The results indicate that the Cape Egmont Fault is currently a baffle to across-fault flow, but sand/sand juxtapositions were present across the fault up until ca. 3.5 Ma before present-day.