Polygonal faulting is commonly found in the shallow overburden rocks above hydrocarbon reservoirs (Figure 1). It is a term used to describe the fracture pattern of hydraulic fractures that can form at shallow depths. They form during rapid deposition of fine grained sediments (muds), which prevents sediment de-watering and consequently the rocks are often over pressured.
Its better to call these polygonal fractures as they can be tensional or shear in origin. If the differential stress (sigma 1 - 3) is < 4T (the tensional strength) of the rock, tensional fractures will form, but shear fractures (faults) will form if sigma 1 - 3 is >4T (Figure 2). If the horizontal stresses are isotropic (similar values), there is no horizontal direction of relatively easy opening and therefore no tendency for fractures to align in a particular direction. Figure 2 helps us understand the genesis of the polygonal pattern in the shallow Cretaceous mudstones above the Clair Field (West of Shetland) in Figure 1. The horizontal stresses are isotopic and the pattern has been created by either interfering tensional fractures (Figure 2c) or by shear fractures (Figure 2d) - we'd be able to tell which by looking at the fracture dip in some seismic sections.
A seismic section through a N Sea overburden section (Figure 3) shows that polygonal faults just look like regular tectonic faults but differ in that they are usually bed-bound. And these are faults due to mappable shear displacements (i.e. sigma 1 - 3 > 4T; Figure 2). Some examples of cored polygonal faults are shown in Figure 4 from the Clair Field (Ogilvie et al. 2015) - again they are shear fractures as slickenlines are observed.
My understanding is that outcrop examples are hard to come by - as these fractures are generally not well preserved as they form at shallow burial depths where they are unlikely to be preserved as veins (Cosgrove, 1998) and fractures would heal once excess fluid pressure has dissipated.
Figure 1. Polygonal Faulting in the Cretaceous mudstones of the Clair Field (illuminated attribute on upper faulted surface). Model for their formation in relation to underlying fault - details in Ogilvie et al (2015). The red spider pattern is a well set.
Figure 2. Different patterns of hydraulic fractures relative to principle stresses. From Cosgrove (1998).
Figure 3. Seismic section through overburden in a North Sea Field showing that polygonal faults look like regular tectonic faults, but are usually bed bound.
Figure 4. Cretaceous core through polygonal faults in the Clair Field, West of Shetland. (a) slickenlines indicating shear in Masstrichtian mudstone (b) bedding parallel shear in a polygonal fault zone in the Campanian. From Ogilvie et al (2015).
References
Cosgrove, J. 1998. The Role of Structural Geology in Reservoir Characterisation. In Coward,M.P., Daltaban, T. S. & Johnson,H. (eds) Structural Geology in Reservoir Characterization. Geological Society, London, Special Publications, 127, 1 13.
Ogilvie, S.R, Barr, D, Roylance, P, Dorling. M. 2015. Structural Geology & Well planning in the Clair Field. From: Richards, F. L., Richardson, N. J., Rippington, S. J., Wilson, R. W. & Bond, C. E. (eds) 2015. Industrial Structural Geology: Principles, Techniques and Integration. Geological Society, London, Special Publications, 421, 197–212.