Viscous Drag During Gravitational Sinking of Brittle Anhydrite Stringers in Zechstein Salt Above the Groningen Field Could Cause Minor Earthquakes

Induced earthquakes due to gas production have taken place in the province of Groningen in the northeast of the Netherlands since1986. Anhydrite layers or stringers are located in the Zechstein salt above the Groningen field. Recent high resolution studies have shown that a few small seismic events are located in in the Anhydrite stringers enclosed in the salt, quite high above the gas reservoir. We ask the question if these events could be caused by viscous drag on the stringers during slow gravitational sinking through the salt, caused by the density difference between the two rocks. For this we model the gravitational sinking of anhydrite inclusions embedded in Zechstein salt which has been tectonically largely inactive during the Tertiary. Our models are inspired by seismic data which image the stringers encased in the salt. The main work is to use and develop techniques in Abaqus to make models of rheological behaviour of salt tectonics and brittle or ductile behaviour of carbonate stringers embedded in salt. Results show that stringer fragments would sink around 30-40m in 60Ma in a salt with power law rheology and this sinking of the large stringers in salt can establish bending stress in the stringer and create locally stress conditions at failure. For example, some stress state and Mohr's circle in stringer moves to the left and gets close to failure line. Results also show that the stringer with smaller thickness and larger width is easier to have bending deformation which causes failure. This would predict that even in the absence of gas production there are rare, sinking related earthquakes in the stringers which can be detected with the very dense seismometer network. Secondly, the gas production leads to a large subsidence bowl (50cm subsidence for the whole 30 km of the Groningen field) and this could increase the stresses in the stringers - the bending moments are in the same direction as the sinking-related ones.