Piercement mechanisms for mobile shales

Abstract

AbstractWe have identified seven mechanisms by which mobile shales can pierce their roofs. The operative piercement mechanism depends on mobile‐shale viscosity, roof strength and stress state. For mobile shales at depths of several kilometres, three mechanisms are possible: fracture piercement, thrust piercement and ductile “piercement.” However, injection up fractures and faults appears to be the dominant mechanism by which mobile shales rise towards the surface. In this process, mobile shales behave similar to magmas rising through the Earth's crust. Nearer the surface, a wider range of piercement mechanisms becomes possible: passive piercement, reactive piercement, active piercement and erosional piercement. These mechanisms all have salt‐tectonics analogues. Although shale tectonics and salt tectonics share common piercement mechanisms, in many cases the resulting structures are different. This is because near‐surface mobile shales can have much lower viscosities than salt. Mobile shales that reach the surface extrude very rapidly, in many cases leading to caldera collapse of the underlying shale chamber. This instability in the near‐surface means that long‐term, stable growth of passive shale diapirs is unlikely, in contrast with the behaviour of salt. A key question in seismic interpretation of mobile‐shale structures is whether large‐volume mobile‐shale diapirs exist. We show that both active piercement and ductile “piercement” can create such structures. Both of these mechanisms create steeply upturned beds on diapir flanks, which are diagnostic. However, active shale diapirs appear to be rare, and ductile “piercements” are not documented. We therefore suggest that large‐volume shale diapirs should be interpreted with caution on seismic data.