A Geomechanics Approach
Natural fractures can provide essential permeability in a reservoir; individual fractures are below seismic resolution and a range of datasets are required in order to characterise naturally fractured reservoirs.
A geomechanics approach can improve the prediction of fracture distribution and intensity. An example is that thicker sedimentary beds often have less fractures than thinner beds [1]. Note how the thick sandstone bed in the upper part of the outcrop in Figure 1 is has little/no fractures whereas the thinner sandstone beds are fractured. In this case these fractures are likely to be joints are they are near vertical and don’t tend to offset the bedding.
The shales are less fractured than the sandstones as the ductile shales are more difficult to fracture. Therefore, mechanical stratigraphy is controlling occurrence of fractures in this outcrop.
Figure 1. Interbedded sandstones and shales, E Beach St Andrews, UK. The sandstones are more highly fractured than the shales (some are shown with red lines on right hand side).
The view on Figure 2 clearly shows jointed sandstones although the shales also appear to have fractures albeit at smaller scale.
Figure 2. Dipping sandstone beds encased in shales; note the high intensity of fractures in the sandstone (further along coast from Figure 1).
Near vertical sandstone beds are shown in Figure 3; again the sandstones are more highly fractured with the deformation occurring pre-folding.
Figure 3. Fe-rich sandstone beds with joints. The surrounding shales are relatively unfractured. The beds are near-vertical, likely forming the steep limb of a fold.
Key takeaways:
It is important to predict the occurrence of natural fractures as many reservoirs rely upon their permeability.
A geomechanics approach is valuable as fracure intensity can often be related to bed thickness where thick beds tend to have fewer fractures than thick beds.
Related to this is mechanical stratigraphy where certain lithologies (in this case sandstone) are more fractured than others (often shale as is the case in this outcrop).
References:
[1] McQuillan, H. 1973. Small scale fracture density in Asmasri Formation, southwest Iran and it’s relation to bed thickness and structural setting. AAPG Bulletin, 57, 2367 - 2385.