35. What are faults and fractures?

Fractures are discontinuities or "cracks" in rocks, caused by stresses within the earth's crust. Fractures are referred to as faults when the rocks on either side of a fracture have moved in different directions, causing displacement (offset) of the rock layers (see Figure 24). Faults and fractures can be simple or complex in geometry, may be open or closed, and therefore can form either channels for fluids to flow, or barriers where fluids get trapped. Oilfields are often created when a fault has helped to trap the oil in place.

Figure 24. An example of the three basic kinds of faults. Image reworked from information provided by the US Geological Survey (http://sound-waves.usgs.gov/2009/11/fieldwork2.html).

An understanding of fractures in a storage reservoir or oilfield can help scientists predict how injected CO2 will spread out and be contained. The nature of any fractures and faults in surrounding rocks needs to be understood so that safe containment of injected CO2 can be assured.

For the WMP, characterisation of the oil reservoir and surrounding rocks was made possible through the collection of data and samples, and extensive research. This has allowed a comprehensive understanding of the distribution and nature of faults and fractures, showing that the existing faults and fractures at the Weyburn-Midale fields would not impair the fields' ability to trap and contain CO2 or other fluids injected into the reservoir, just as the oil and gas have been contained for millions of years.