Figure 1: Process of hydraulic fracturing (NL = 'hydraulische fracturatie')
Figure 2: The Rursee in the North Eifel, Germany. PhD study site with excellent Lower Devonian Geology & tons of veins.
During my PhD research, the Lower Devonian sandstones exposed in the Belgian Ardennes and German North Eifel mountains were studied as they host numerous quartz vein types that were created by hydraulic fracturing at the onset of the Variscan Orogeny (325-300 Ma). The present quartz mineral veins represent sealed fluid pathways and contain valuable information on temperature and pressure changes, and on stress patterns present during fluid flow. Mapping the geometrical interaction of veins with elements such as bedding, cleavage and folds allowed deciphering the precise timing of vein emplacement with respect to deformation. Pressure and temperature (p-T) conditions of veining/fracturing were derived from micro-thermometry of fluid inclusions present in the vein quartz.
Fig. 3: Research methodology
Fig 4: Studied vein types in the High-Ardenne slate belt
Fig. 5: P-T conditions of the two vein types deduced
from fluid inclusion microthermometry
Fig. 6: Studied sites and reconstruction of the stress-state at the time of bedding-normal veining after refolding veins to their original position
Results of this combined research approach showed that elevated (supra)lithostatic fluid pressures are easier to maintain during tectonic inversions, i.e. the stress-state change from extension to compression or vice-versa, than during the main extensional phase of a basin or the contractional phase of an orogeny. In the study area, bedding-normal veins were developed prior to inversion and bedding-parallel veins after inversion. Tectonic inversions are thus crucial timings during which overpressured fluids can be maintained at a regional scale in which the remote state of stress controls the geometry of rock failure. An important discovery was that these coupled changes in stress-states and fluid pressures lead to triaxial stress inversions (Van Noten et al. 2012) and are more complex than the biaxial stress changes in theoretical models. This interaction is important for understanding the genesis of ore deposits as they follow preferential pathways created by the fracture network.
Figure 7: Stress-state evolution of the brittle upper crust illustrated in a brittle failure mode plot. Horizontal scale bar illustrates the differential stress changes during orogeny. Vertical scale bar illustrates the changes in pore-fluid pressure during orogeny. Bedding-normal veins are formed at near-lithostatic pressures. Bedding-parallel veins are formed at supra-lithostatic pressures, able to 'lift up' to rock column.
Figure 8: Thin section of a bedding-parallel quartz vein illustrating the complexity of its formation history: phases of uplift and illustrated by fiber & blocky quartz crystal growth alternated with phases of bedding-parallel shear shown by shear laminae with fine-grained crystals.
The discoveries of my Ph.D. are highlighted in six first author papers cited in various fields. The Van Noten & Sintubin (2010) paper, reporting on the relationship between fracture spacing and bed thickness, influences mechanical fracture development studies. My 2008, 2011 and 2012 papers are cited in numerical and laboratory vein analogue studies and in ore geological studies. As hydraulic fracturing is artificially induced in reservoir exploitation to increase permeability in natural reservoirs, the performed fluid inclusion analysis is particularly useful for a reliable estimate of the p-T conditions present in the seismogenic crust.