Piercement mechanisms for mobile shales

Abstract

We 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.

Extensional Deformation of a Shale-dominated Delta: Tarakan Basin, Offshore Indonesia

“…Given these observations, we suggest that mud volcanoes are formed by reactive diapirism, with mobile shale ascending along sub-seismic fractures developed above buried, seismically imaged normal faults (e.g. Hudec and Soto, 2021).…”

“…1b), suggesting they formed in response to overburden extension due to gravitational failure of the deltaic wedge within which they developed (e.g. Morley, 2003;Soto et al, 2010;Hudec and Soto, 2021), and/or extension driven by differential compaction and fluid expulsion (e.g. Van Rensbergen and Morley, 2000;Back and Morley, 2016).…”

“…Damuth, 1994;Morley and Guerin., 1996;Cohen and McClay, 1996;Briggs et al, 2006, Santos Betancor andSoto, 2012;Zhang et al, 2021). However, difficulties with seismically imaging remobilized shale bodies mean that we have a poor understanding of the shape and size of these features, and the mechanisms (e.g., brittle vs. ductile) driving deformation (see Hudec and Soto, 2021). For example, previous 2D seismic-based studies identify chaotic seismic facies interpreted to reflect thick, mobile shale and overlying, shale-detached listric normal faults (e.g.…”

Extensional deformation of a shale‐dominated delta: Tarakan Basin, offshore Indonesia

“…Damuth, 1994;Morley and Guerin, 1996;Cohen and McClay, 1996;Briggs et al, 2006;Espurt et al, 2009;Morley et al, 2011;Santos Betancor and Soto, 2012;Rowan, 2020;Zhang et al, 2021). However, difficulties with seismically imaging mobilized shale bodies mean that we have a poor understanding of the shape and size of these bodies, and of their mechanisms (e.g., brittle vs. ductile) and involved deformation (see Soto, 2021 andSoto et al, 2021a). For example, Van Rensbergen and Morley (2000; use 2D seismic data to document chaotic seismic facies, being interpreted to reflect thick, mobile shale in footwall of shale-detached listric normal faults.…”

Oscillations of a particle-laden fountain

“…Particles entrained by fluid motion are not only a fascinating topic for physicists, but are also at the core of present-day challenges in geosciences or in industrial processes. Fluid migration in sedimentary basins and their subsequent expulsion at the seafloor lead either to gentle fluid release and the formation of pockmarks [1,2], or to violent subsea fluid release such as mud volcanoes [3]. These latter can exhibit puzzling periodic behaviors [4].…”