14 Wrap-up
This Groundwater Project book introduces the different kinds of fractures that occur in siliciclastic rocks, primarily in sandstone and with some consideration of the presence of interlayered shales, and the effect of these fractures on the flow of geofluids. The major fracture types and their occurrence in groups are described, and a few other classes that are considered rare in nature are mentioned. The references listed in this book provide more detailed information on this topic.
The overlying messages are that fracture types are widespread in siliciclastic rocks and these structural elements have a significant impact on groundwater flow. Almost every groundwater and contaminant transport problem in sandstone aquifers will involve complexities. It is best to be aware of them rather than assume the medium is homogeneous. The good news is that the heterogeneities imparted by these “fracture” features have been well studied at prototypical locations, which represent common tectonic and geological settings. The bad news is that other groundwater aquifers and their tectonic history are most likely different from those presented in this book. Therefore, the application of the results highlighted here to a specific groundwater environment may not be straightforward. However, understanding the deformation processes responsible for common fractures as well as their distribution and variability is useful when designing a strategy to deal with such issues in other settings.
There is a strong connection between structural heterogeneities and depositional heterogeneities including the most common one, bedding, particularly in the presence of fine-grained deposits such as shale or mudstone along the bedding interfaces and interdune boundaries in siliciclastic rocks. Most of the time, these cannot be resolved by seismic data or by investigating boreholes. Drilling is expensive so boreholes are sparse. That is why gathering basic knowledge of rock fracture from outcrops is one of the most affordable methods of assessing fracture heterogeneities and their potential impact on flow properties. When boreholes and/or seismic reflection data are available, the correlation of data from one borehole to another often hundreds of meters (sometimes several kilometers) apart or from one seismic reflection line to the next similarly far away is enhanced by knowledge about the potential fracture types, their architecture, their hierarchy, scaling relationships and petrophysical properties. It is a challenge to deal with these fracture properties in complex underground environments. This book provides examples of how to break down complex fracture systems by deciphering their components and how to understand the fracture systems by using the mechanical processes responsible for their progressive formation. It helps to keep in mind that everything in nature happens for a reason. It is up to the interpreter to discern the underlying processes that fractured the siliciclastic aquifer of interest. Thus, this book provides basic knowledge of fracturestructures and their relationship to depositional fabric and mechanical processes that resulted in their formation.
Studies based on 3D and 4D modeling approaches are essential to the understanding and prediction of fracture and fault networks and should be used as soon as technology improves and new analysis tools become available. Such models would help in understanding not only fracture within small areas but also large-scale landscapes (Boersma et al., 2020).
Working with faults and fractures in geology applied to water flow and groundwater systems can be challenging because it is necessary to work with multiple mapping techniques and to call on more than one technical field such as mapping, structural geology, geochemistry and petrology (Laubach et al., 2018).