Proposal for a mechanical model of mobile shales

Abstract

Structural systems involving mobile shale represent one of the most difficult challenges for geoscientists dedicated to exploring the subsurface structure of continental margins. Mobile-shale structures range from surficial mud volcanoes to deeply buried shale diapirs and shale-cored folds. Where mobile shales occur, seismic imaging is typically poor, drilling is hazardous, and established principles to guide interpretation are few. The central problem leading to these issues is the poor understanding of the mechanical behaviour of mobile shales. Here we propose that mobile shales are at critical state, thus we define mobile shales as “bodies of clay-rich sediment or sedimentary rock undergoing penetrative, (visco-) plastic deformation at the critical state”. We discuss how this proposition can explain key observations associated with mobile shales. The critical-state model can explain the occurrence of both fluidized (no grain contact) shales (e.g., in mud volcanoes) and more viscous shales flowing with grain-to-grain contact (e.g., in shale diapirs), mobilization of cemented and compacted shales, and the role of overpressure in shale mobility. Our model offers new avenues for understanding complex and fascinating mobile-shale structures.

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

“…Although we cannot conclusively resolve the exact nature of the processes mobilizing the shales, we speculate that the smectite-transformation, in combination with increasing shear stresses by normal faulting, could have jointly participate in creating overpressure conditions in this shaly unit (Soto et al, 2021a;Li et al, 2022). Rapid sedimentation of this clay unit likely lead to fluid entrapment, making it possible to achieve the critical-state conditions to permit essentially solid-state flow at relatively lower shear stresses.…”

“…Chima et al, 2022), the existence of fluid expulsion structures (Back and Morley, 2016), the occurrence of a base-shale relief (Chima et al, 2022), the deep flow of mobile shale under either brittle or ductile conditions (e.g. Cohen and McClay, 1996;Soto et al, 2021a), and the growth history of supra-shale faults (e.g. Fazlikhani and Back, 2012).…”

Shale mobility: From salt-like shale flow to fluid mobilization in gravity-driven deformation, the late Albian–Turonian White Pointer Delta (Ceduna Subbasin, Great Bight, Australia)

“…Oil and gas generation and migration most probably occurred much earlier within the deeper, gas prone synrift sequence, thus leaving time for gas to migrate and reach the shale plugs and withdrawal minibasins undergoing 9 shortening above the main thrust fault. Gas generation has a strong impact on the mechanics of shale, promoting the complete loss of cohesive resistance (Tingay et al, 2018;Blouin et al, 2020), a mechanism which may help shales to achieve critical state and enhance mobility (Soto et al, 2021a), eventually leading to fluidization and through fracturing processes, to loss of material.…”

“…has long been debated, following developments of deep offshore seismic acquisition on the one side, improvement of seismic imaging and processing techniques on the other (Morley and Guérin, 1996;Soto and Hudec 2021;Soto et al, 2021a, b, among others). Understanding of shale mechanics helped to set the ground for understanding the ability of already compacted and cemented shales to behave in a viscous manner (Brown, 1990;Soto et al, 2021a) and salt-like shale tectonics was put at the forefront in various settings recently (e.g. Hudec and Soto 2021;Dinc, 2020;Back and Morley 2016;Ruh et al, 2018).…”

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.…”