Paolo Pace (Pace Geoscience) and Steven Ogilvie (Ogilvie Geoscience)

Natural fractures can provide the essential permeability in a hydrocarbon and geothermal reservoir. However, they exist in the sub-seismic domain and although they can be detected by well logs and cores, we are reliant upon outcrop analogues to understand their distribution and connectivity in 3D. We can use a simple geomechanics approach to predict their distribution and intensity as a basis for our conceptual model.

In general, thicker beds tend to have less and more widely spaced fractures than thinner beds. Note how the thicker sandstone bed in the upper part of the outcrop (Fig. 1) is essentially unfractured whereas the thinner sandstone beds have regularly spaced, vertical joints. The geomechanics reason often given is that tall fractures in thick beds tend to cast stress shadows that prevent new fractures from initiating close by. A simpler explanation is that thinner materials will in general break more easily than thicker materials. If the area/field of interest has little data, this is a concept that can be carried together with other concepts such as distance to major faults until such time that the data exists to reduce the number of scenarios.

The other observation we can make from the photograph (Fig. 1) is that the shales are more poorly fractured than the sandstones. This is because shale is usually more ductile than sandstone and when subjected to the same amount of strain, they will take longer to break. Sometimes, fractures enter the shales at a lower angle which is consistent with propagation through a rock of lower frictional strength.

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 outcrop view below (Fig. 2) shows jointed sandstones although the shales also appear to have fractures albeit at smaller scale.

Figure 2. Dipping sandstone beds alternating with shales; note the high intensity of fractures in the sandstone (further along coast from Fig. 1).

Similar concepts of bedding-related mechanical stratigraphy can also be observed in carbonate sedimentary multilayers. The outcrop example of interbedded limestones and cherts shows a major contrast in the distribution of fracture intensity within the well-layered package (Fig. 3). It can be observed that each layer interface represents a sharp discontinuity that marks a lithological change acting as a mechanical boundary for fracture propagation. In such a mechanical pattern, fractures (mostly open-mode joints) are prone to be bed-confined and the fracture intensity and distribution are a function of the mechanical properties of the different carbonate layers.

Thicker coarse-grained limestone (calcarenite) beds have a lower fracture intensity (red lines, Fig. 3) than the thinner fine-grained limestone (micrite) beds (green lines, Fig. 3). This is particularly the case when these beds are adjacent or interbedded with chert layers or lenses. Also indicated in Fig. 3 is a meso-scale throughgoing fault (blue), which we interpret as a tilted syn-sedimentary normal fault (note stratal thickening to the right).

Figure 3. Interbedded limestones and cherts (Pennapiedimonte, Italy) of Upper Cret. age; the thinner micritic limestone beds are most fractured. Outcrop example from the Majella anticline in the foreland fold-and-thrust belt of the Central Apennines, a world-class analogue for the study of fractured carbonate reservoirs.

Fig.3 features in the “nothing beats the field” series in GeoExpro magazine

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Key takeaways:

It is important to predict the occurrence of natural fractures as many reservoirs rely upon their permeability for well deliverability.
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 are more fractured than others.

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.