Finding the faults in caprocks
Our research has allowed oil and gas companies to understand how faulted caprocks behave under specific geological conditions.
Caprocks are important for resource exploitation because they have intrinsically low permeability, hindering the movement of fluids in the subsurface and providing potential barriers for the escape of resource or toxic fluids to the Earth's surface.
Faults and fractures in rocks are universally attributed to tectonic stresses. In 1994, Professor Joe Cartwright identified a fault system that didn't conform to this standard model. These faults were characterised by their polygonal pattern, similar to shrinkage cracks on a dried-out lake bed, albeit on a much larger scale.
Our research focused on establishing the effect of these faults on the integrity of caprocks and the compartmentalisation of reservoirs. After a first stage largely funded by the hydrocarbon Industry and the Natural Environment Research Council (NERC), research focused on the development of models for the formation of polygonal faults. Following the award of a NERC grant on the genesis of polygonal faults, efforts concentrated on recognition of the timing of fault formation.
This led to the discovery of significant controls on the reactivation of early faults on both reservoir and caprock intervals and, later, on the recognition of an important degree of internal organisation in blocky and faulted submarine mass-flows, which are commonly part of caprocks.
Our work has now demonstrated conclusively that faulted and blocky caprock intervals have important implications for the degree of connectivity between reservoir units, and to the seal competence of caprocks.
In the petroleum industry, caprock is any non-permeable formation that may trap oil, gas or water, preventing it from migrating to the surface. This caprock or trap can create a reservoir of oil, gas or water beneath it.
Direct impact on industry
The value of the research on seal quality was recognised by The Royal Society and the Wolfson Foundation, which invested £200K in a new laboratory for CO2 sequestration in Cardiff in 2008. This research has also led to new NERC-funded studentships and, since 2012, to new consortia led by Tiago Alves. Most of these consortia investigate distinct stratigraphic architectures and styles of compartmentalization in reservoir intervals.
Our research has also had a direct impact on the global petroleum industry. This has led to economic gains, through a more effective approach to oil and gas exploration and extraction, building on more effective risk analysis.
It's also meant changes in professional practice in the petroleum industry, including a new approach to the risk analysis of caprock seal integrity. Lastly there have been environmental benefits, through improved location of sub-surface gas accumulations, and identifying potential CO2 sequestration sites.
- Alves, T. M. et al. 2014. Assessing the internal character, reservoir potential and seal competence of mass-transport deposits using seismic texture: a geophysical and petrophysical approach. American Association of Petroleum Geologists 98 (5), pp.911-945. (10.1306/09121313044)
- Alves, T. M. 2012. Scale-relationships and geometry of normal faults reactivated during gravitational gliding of Albian rafts (Espírito Santo Basin, SE Brazil). Earth and Planetary Science Letters 331-2 , pp.80-96. (10.1016/j.epsl.2012.03.014)
- Cartwright, J. A. 2011. Diagenetically induced shear failure of fine-grained sediments and the development of polygonal fault systems. Marine and Petroleum Geology 28 (9), pp.1593-1610. (10.1016/j.marpetgeo.2011.06.004)
- Alves, T. M. 2010. 3D Seismic examples of differential compaction in mass-transport deposits and their effect on post-failure strata. Marine Geology 271 (3-4), pp.212-224. (10.1016/j.margeo.2010.02.014)
- Alves, T. M. , Cartwright, J. A. and Davies, R. J. 2009. Faulting of salt-withdrawal basins during early halokinesis: effects on the Paleogene Rio Doce Canyon system (Espírito Santo Basin, Brazil). AAPG Bulletin 93 (5), pp.617-652. (10.1306/02030908105)