Fracture Geometries Associated with Sub-Seismic Faults: Outcrop Analysis, Modeling and Prediction, from Example from the Chalk of Southeast England
The Cretaceous chalk exposed along the coast of Kent, southeast England, provides an analogue for many producing North Sea reservoirs. Mode I dilatant fractures are an important component of these reservoirs, increasing the permeability in the brittle and low permeability chalk. Coastal outcrops provide an opportunity to examine the controls on fracture distribution and geometry as well as evaluating the influence of (sub-seismic) faults on fracturing.
The chalk is well exposed in vertical cliffs and a wave cut platform on a 500m coastal section at Pegwell Bay, Kent. The section contains a series of small sub-parallel strike-slip faults, typically ~10m apart with throws <5m. It is also heavily fractured, and the fractures are observed to cluster around the faults. Two styles of fracturing were noted:
a) Concentric rings of fractures which formed around bends and tips in the strike-slip faults. These fractures are typically short and localised (they rarely extend >1m from the faults), and thus are unlikely to contribute significantly to production.
b) Fracture corridors, comprising thin (typically 1-2m wide) zones of closely spaced parallel fractures, extending in a straight line over long distances (typically 10-50m). They usually start at bends or tips in the faults, and often connect to the adjacent faults. They are thus likely to have a significant impact on the flow of fluids through the chalk.
To understand how slip on the faults could influence fracture distribution and geometry, we used the elastic dislocation model of Okada (1992) to calculate the local stress perturbations formed around bends and tips in strike-slip faults under different conditions. We compared the resulting stress orientations with the fracture patterns mapped in the field and were able to account for both the concentric fractures and the fracture corridors. The models predict zones of low minimum stress around both fault bends and fault tips, which can explain the origin of the fracture corridors at these points. These results emphasise the importance of the minor faults in controlling the distribution and geometry of fractures in the chalk.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009