Properties of Polygonal Fault Systems - Fluid Flow, Aligned Arrays and Simple Shea

Dedicated (Fluids and Fluid Flow). Layer-bound fault systems are found in sequences of fine-grained sediment that have undergone passive subsidence and burial. Where bedding is near-horizontal, these faults cut bedding planes in polygonal geometries. Laboratory measurements of clay properties and effective stresses estimated from well data are consistent with the hypothesis that low coefficients of residual friction in fine-grained sediments are fundamental to the development of such fault systems. Pipes and pockmarks above the upper tips of such faults are evidence of episodic fluid expulsion, which probably results from compaction of the host sequences, in general, and not transmission through them. One exception is where polygonal faults are associated with injectites identified in Eocene claystones of the Central North Sea. Circumstantial evidence suggests that the injectites have acted as migration pathways for hydrocarbons. Further evidence of fluid expulsion through polygonal faults is provided by the extraordinary topography of the opal-A/opal-CT reaction zone found around the upper tips of polygonal faults. Where the regional dip exceeds ~1, the faults are aligned in parallel arrays. On the Mauritanian margin, antithetic faults dip less steeply than synthetic faults. This observation implies that substantial, gravity-driven, simple shear of the host sediments has occurred.