The effect of salt in dilatant faults on rates and magnitudes of induced seismicity – first results building on the geological setting of the Groningen Rotliegend reservoirs

Abstract

The presence of salt in dilatant normal faults may have a strong influence on fault mechanics in the Groningen field and on the related induced seismicity. At present, little is known of the structure of these fault zones. This study starts with the geological evolution of the Groningen area, where, during tectonic faulting, rock salt may have migrated downwards into dilatant faults. These fault zones therefore may contain inclusions of rock salt. Because of its rate-dependent mechanical properties, the presence of salt in a fault may introduce a loading-rate dependency into fault movement and affect the distribution of magnitudes of seismic events. We present a first-look study showing how these processes can be investigated using a combination of analogue and numerical modelling. Full scaling of the models and quantification of implications for induced seismicity in Groningen require further, more detailed studies: an understanding of fault zone structure in the Groningen field is required for improved predictions of induced seismicity. The analogue experiments are based on a simplified stratigraphy of the Groningen area, where it is generally thought that most of the Rotliegend faulting has taken place in the Jurassic, after deposition of the Zechstein. This suggests that, at the time of faulting, the sulphates were already transformed into brittle anhydrite. If these layers were sufficiently brittle to fault in a dilatant fashion, rock salt was able to flow downwards into the dilatant fractures. To test this hypothesis, we use sandbox experiments where we combine cohesive powder as analogue for brittle anhydrites and carbonates with viscous salt analogues to explore the developing fault geometry and the resulting distribution of salt in the faults. Using the observations from analogue models as input, numerical models investigate the stick-slip behaviour of fault zones containing ductile material qualitatively with the discrete element method (DEM). Results show that the DEM approach is suitable for modelling the seismicity of faults containing salt. The stick-slip motion of the fault becomes dependent on shear loading rate with a modification of the frequency-magnitude distribution of the generated seismic events.

Geology of the Groningen field – an overview

“…Factors contributing to the sealing capacity of faults include: fault throw, cataclasis and clay smearing, size of juxtaposition window and fault orientation relative to present-day maximum stress direction. Ketterman et al (2017) conclude from sandbox and numerical modelling that Zechstein salt may have flowed downward into opening fractures and faults, possibly contributing to local fault sealing. It has been attempted to assess the role of fault type (normal versus inverse, dip-slip versus strike-slip), and of timing and fault reactivation history, but no relation was found.…”

A review of source models to further the understanding of the seismicity of the Groningen field

“…Nevertheless, the magnitude and orientation of stress as a function of space and time is often least known, since stresses cannot be measured directly with remote sensors and are difficult to determine in situ. Another crucial point in the development of more accurate fault models is the insight that salt intrusions within the faults may affect dynamic ruptures significantly, because they may introduce a rate dependency to fault movement and change the frequency-magnitude distribution of seismic events (Kettermann et al, 2017). Although for Groningen, there are currently no subsurface data indicating the existence of salt-filled faults, this may only be due to technical limitations.…”

“…Although for Groningen, there are currently no subsurface data indicating the existence of salt-filled faults, this may only be due to technical limitations. Using analogue models, Kettermann et al (2017) demonstrated that salt from the Zechstein formation might flow down-dip into opening faults due to gravitational flow.…”