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Influence of Pre-Existing Fabric from Normal Fault Segmentation and Hydrocarbon Systems

Paton, Douglas 1; Mortimer, Estelle 1
1 School of Earth and Environment, University of Leeds, Leeds, United Kingdom.

The presence of pre-existing fabrics or structures within the continental lithosphere plays a fundamental role on how normal faults grow and interact. Understanding, and being able to predict, the nature of this growth has important implications when considering both source rock and reservoir distribution within extensional provinces.

Two end-members of structural inheritance are presented: where the extensional direction is approximately perpendicular (high alpha) to the existing structural fabric; and where the extensional direction is close to parallel to the fabric (low alpha). Using onshore and sub-surfaces examples from South Africa and the East African Rift, we present evidence for both of these end members and demonstrate a significant difference in individual normal fault growth and interaction. In cases of high alpha, faults segments are aligned, there is little or no evidence of seismic scale en-echelon architecture and faults achieve significant longer than predicted lengths early in the syn-rift. This results in fewer relay-ramps, hence simpler and axially dominated sediment pathways. In addition, these long early faults may have increased subsidence associated with the fault linkage and therefore may be more prone both to fault scarp degradation and to increases in hanging-wall subsidence resulting in earlier anoxic conditions with potential for source rock deposition

In contrast, in settings of low alpha values, fault segments tend to be shorter, with significant amounts of fault tip segmentation. This results in overall lower displacement, shorter faults that have a greater number of relay ramps. The consequence of this style of linkage is that there are more sediment entry points to a greater number of depocenters and correspondingly a more convoluted sediment pathway. In addition, the lower displacement leads to shallower water, hence lower likelihood of anoxic, source rock deposition.

The influence of segmentation may be difficult to define, in particular in frontier areas where line spacing does not enable high resolution definition of fault segmentation. The models discussed here provide a predictive tool for determining the geometry of fault segmentation, but also how that geometry is likely to influence both syn-rift source rock and reservoir fairway distribution.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009