Coastal exposures of naturally fractured sandstones/siltstones are found on the far N coast of Scotland (St Mary’s Chapel tag opposite).
Looking down on the wave-cut platform, different orientations of joints have created a grid-like pattern (Fig. 1). These outcrops are analogues for the giant Clair oil field, offshore West of Shetland.
Figure 1. Open fractures/joints in Devonian sandstones, Caithness, UK to illustrate the nature of the fracturing penetrated by wells in the Clair field. St Mary’s Chapel, Caithness (GR ND 025 702). Photograph courtesy of Alex Milne, published in Ogilvie et al. (2015).
In section view, the impact of bed thickness on fracture density can be seen - although it is difficult to see this on Fig 2. Thinner beds often have a higher density of natural fractures than thicker beds. I’d recommend considering this driver during fracture modelling. We can explain this driver by assuming that the width of the stress shadow (release of energy during fracturing) is a function of the height of the fracture. Taller fractures cast a wider stress shadow, forcing fractures to form at a wider spacing in thick beds.
The joints can be viewed in 3D (Fig. 2, 3) showing that they do not displace the beds. Also observed are thrust faults which have created small fault propagation folds indicating that there has been some transpression (Fig. 3). Larger scale examples of these structures can trap hydrocarbons, resulting in successful oil field developments.
The fold on the right hand side has a steep forelimb and gentle backlimb and the causal fault appears to have a splaying geometry, resulting in dense fracturing in the forelimb (Fig. 3).
Figure 2. Coastal outcrop of Caithness flagstones (St Mary’s Chapel) containing joints and folds.
Figure 3. Interpretation of area represented by inset in fig 2 - A = thrust faults and associated folds, B = jointing.