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.
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Introduction
Mobile shales are bodies of highly sheared shale that lack coherent reflections in seismic images. Mobile-shale structures range from surficial mud volcanoes to deeply buried shale diapirs and shale-cored folds (Fig. 1a–e). Although they exist in all tectonic regimes (Fig. 1f), they are mostly found in shortening settings (40%) or delta systems on continental margins (31%) (detailed information in Supplementary Information 1).
Mobile shales create challenges for seismic processing, seismic interpretation, drilling, and geohazards analysis. A primary source of uncertainty in interpreting these structures is a poor understanding of the mechanics of mobile shale. Although mobile shales appear to deform through some sort of ductile flow that strongly modifies previous structures like fractures, dissolution and precipitation micro-structures, and even the shape and dimensions of the voids2,3,4,5,6,7, we have little insight into how shales become mobile and flow. Most authors suggest that shales become mobile simply by becoming highly overpressured8,9,10,11, but this simple explanation fails to account for the full range of mobile-shale occurrence. For example, shales (defined broadly, sensu Aplin et al.12) can apparently become mobile even after significant burial, consolidation, and cementation13, 14. There are also examples of organic-rich shales that can form complex flow structures under contraction, even though they have low porosity15. Overpressure can mobilize unconsolidated mud, but an increase in pore pressure alone cannot cause a cemented shale to flow.
Our lack of a viable mechanical model for mobile shale is due partly to the difficulty in sampling subsurface mobile shales and measuring their mechanical properties. Some information can be extrapolated from wells penetrating clay-rich shear faults in seismogenic zones and oceanic-wedge detachments16, 17. Mud volcanoes have been extensively studied18,19,20,21,22, and progress has been made in studying plastic flow of the extruded sediments using sampling observations23,24,25,26,27 and some drilling data28,29,30,31. Nevertheless, we have little knowledge of the physical properties of plastically flowing shales in the subsurface and even less knowledge of the mechanical processes by which they become mobile or stop moving.
This uncertain understanding is reflected in the existing definition of mobile shale. The term was introduced by Morley and Guerin4, who defined them as “any shales deforming complexly by a combination of ductile deformation and brittle failure in the presence of a fluid phase.” This definition was an excellent starting point because it recognized the roles of lithology, deformation style, and fluids in shale mobility. However, it suffers from several shortcomings. First, the definition is overly broad because it includes almost any type of deformation involving shales. Second, it provides little insight into what makes a mobile shale mobile or how it deforms.
In this paper, we summarize observations concerning mobile shales and, using concepts coming from soil mechanics and available information on the mechanics of consolidated shales, we then propose a mechanical model for mobile shales. We explore the implications of this model and its answers to key questions in mobile-shale research: How do shales become mobile? Is shale mobilization a brittle or ductile process? Why do some mobile shales become immobile? Can shales with diagenetic cements become mobile? Does depth of burial play a role in shale mobilization? Do all mobile shales have to be highly overpressured? If so, how high must the overpressure be? How does shale mobilization affect its seismic properties?
What do we know about the mechanics of mobile shales?
There are some observations about mobile shales that are widely accepted. We use these observations as a framework to suggest a mechanical model for mobile shales.
First, the main difference between salt and sedimentary rocks like shales is their contrasting mechanical behaviour (Fig. 2). Salt can experience large-scale deformation by creep at stresses below its yield strength32. By contrast, only limited subcritical (pre-peak strength) creep deformation is documented to occur in shales, although creep seems to be more important in clay- and organic- rich shales33. Creep in shales is promoted by particle dissolution, grain rotation and sliding, pore reduction, and the expulsion of fluids from pores and the clay-bound water34, 35. Most of the deformation in shales occurs after stresses surpass peak strength.
Secondly, there are two distinct types of mobile-shale structures7. The first occurs when the shale behaves as a fluid suspension without grain-to-grain contact36,37,38,39,40. Because of their low viscosity (ca. 101–106 Pa s26, 27), this type of mobile shales forms smaller features like mud volcanoes (Fig. 1a,b) in which the shale moves at high velocities (up to tens of meters per second)20,21,22, 41. In the second type, shale behaves as a viscous-plastic solid involving brittle and ductile fracturing, and grain-to-grain frictional flow5, 6, 8, 15, 42. Because of their high viscosity (ca. ≥ 1015 Pa s43), this type of mobile shale forms large-scale bodies like shale diapirs (Fig. 1c–e) that move at lower velocities than in mud volcanoes.
Third, bedding and other fabrics in mobile-shale structures are strongly disrupted, recording large and extensive deformation, and possibly fracturing. This disruption is one factor causing the loss of seismic signal in mobile shale (Fig. 1c–e)11, 14, 15, 30, 44,45,46.
Fourth, mobile shales are typically associated with high overpressures; i.e., there is a large difference between the pore pressure in the rock and the hydrostatic pressure gradient28, 29, 31. Overpressure is obvious in the fluidized material erupted from mud volcanoes, where there is almost no contact between grains18,19,20, 22, 23. High overpressure in other types of mobile shales such as shale diapirs can be inferred from the low seismic velocities of these bodies4, 7, 14, 30, 45. Most researchers attribute these overpressures to a combination of increase in the volume of the pore fluid (due for example to hydrocarbon generation and cracking, diagenetic transformations, or thermal expansion) and in total compressive stresses47, 48. For example, based on the abundance of methane and other gases (e.g., CO2, N2, other alkanes, He, Rn) expelled during mud-volcano eruptions18,19,20,21,22,23, 41, 49, 50, the mobility of shale in mud volcanoes is also attributed to hydrocarbon transformations and the depressurization of a gas-charged source layer.
Fifth, given the stratigraphic position of blocks ejected from mud volcanoes, some mobile shales are sourced from depths of 9 to 10 km20, 22, 41, 51, 52 (detailed information in Supplementary Information 1). Units sourced from these depths were cemented prior to incorporation in mud volcanoes. This observation represents a challenge for workers in mobile shales, because it is difficult to explain how these cemented blocks were transported.
Any viable mechanical model for mobile shales must be able to explain and be consistent with these observations: fluidization and plastic flow of shales, disruption of fabrics, existence of high overpressure, and mobilization of cemented units.
Mechanical model for mobile shales: deformation at the critical state
Field observations of subsurface mobile shales are scarce owing to the understandable reluctance of drillers to penetrate them29. The model that we propose for mobile shales is based on experimental deformation of shale shear behaviour. Because these tests are typically conducted at low stresses and on poorly lithified soils, they do not directly mimic subsurface conditions. The principles governing the mechanical behaviour of soils53, 54 have also been used to analysed the behaviour of consolidated shales, although important differences exist between the mechanical behaviour of soils and shales55,56,57. According to these studies, shales differ from soils in that they have a major cohesion (stiffness), develop some degree of cementation and anisotropy, and their mechanical characteristics change with depth and temperature (detailed information in Supplementary Information 2, Table s2 and Figs. s2–s5). Principles of soil mechanics have also begun to be extrapolated to depths at which diagenetic transformations operate in shales so that the mechanical behaviour of cemented shales deformed under contraction might be modelled8, 24, 25, 58,59,60,61.
We use the behaviour observed in laboratory tests to infer the mechanical behaviour of mobile shales. In our description, the effective stress (σʹ) is the total stress (σ) less some portion of the pore pressure (u), following the Terzaghi’s equation:
The Biot’s pore pressure coefficient (α) is usually assumed to equal 1 in soft and unconsolidated muds, although in low porosity shales, α lies between 0.3 and 0.9, decreasing with increasing stress48, 62.
Figure 3 illustrates the total and effective stress paths and the stress–strain response obtained from an undrained triaxial test on Norrköping clays63 (Fig. 3). In these tests, the clay sample, retrieved from a core, was first consolidated under uniaxial-strain condition to the in situ vertical effective stress (point 2, Fig. 3a). This represents the loading on the clay as it was buried to its final depth. Then, the sample was compressed horizontally in undrained conditions; during this period, the total vertical stress (σvʹ) was kept constant (path from 2 to 4, Fig. 3a). This represents the loading that the clay would undergo if it was in a region with horizontal shortening. The stress path in this test represents the dominant stress path in fold cores, which is a common setting for mobile shales (Fig. 1f). The test was conducted under undrained conditions because shales have very low permeability, therefore, pore fluid could barely drain out of buried shales during shortening.
The clay response during undrained shortening includes two distinct phases (path from 2 to 4, Fig. 3b). First is a period of strain hardening (path 2 to 3), during which deviatoric stress (q, ordinate axis in Fig. 3a) increases as the clay is compressed horizontally and the shear deformation increases. During this period, the effective mean stress (p′, abscissa axis in Fig. 3a) decreases. The sample tends to compact as it is sheared, but this compaction is prevented by pore fluid that cannot escape, leading to an increase in pore pressure (shear-induced overpressure48, 62) and decrease in effective mean stress. The strain-hardening phase occurs at relatively low strains, until the peak strength of the sample is achieved and continuous fractures are formed (point 3 in Fig. 3b).
The second stage, after peak strength, consists of strain-weakening behaviour (path 3 to 4, Fig. 3b), during which deviatoric stress (q) decreases. Softening is attributed to penetrative fracturing and the collapse of rock fabric or breakage of cementation/bonds, also known as destructuration64 (Fig. 4c). Destructuration entails elevated rock compression, which, in undrained conditions, translates into a significant increase in pore pressure (uD, Fig. 3a–b) and decrease in effective mean stress54, 65.
The clay reaches a state where stresses almost stop changing (point 4, Figs. 3, 4), even though the sample is still being shortened and deformed. This is the critical state, at which unlimited (plastic) shear deformation occurs without any changes in stresses or volume53, 69, 70 (plateau, Fig. 3b).
In a perfectly homogeneous medium, materials at the critical state flow everywhere, destroying all material fabrics and cement. In the real world, geologic materials are never perfectly homogeneous. Weak zones (like organic-rich domains and regions with authigenic quartz cements68, 67) fail first, producing an anastomosing network of highly sheared rock surrounding lenses in which earlier structures are preserved5, 6, 71, 72 (Fig. 4).
Vane tests show that at critical state the clay flow is viscous, that is, the strain rate at critical state varies with shear stress (Fig. s6 in Supplementary Information 2). As such, the clay behaves as a Herschel–Bulkley material: it is solid (no flow) when shear stress is smaller than the static shear strength, and when shear stress exceeds the strength, it flows as a fluid material at a strain rate that increases with the excess shear stress (Fig. 2).
Destructuration of sedimentary and tectonic fabrics, breaking of cement, large shear deformation, high overpressure, and plastic flow associated with the critical state tie this state to mobile shales. We therefore propose that mobile shales are at the critical state and suggest the following definition for mobile shales: “bodies of clay-rich sediment or sedimentary rock undergoing penetrative, (visco-) plastic deformation at the critical state”.
Implications of a critical-state model for mobile shales
Fluid-supported versus grain-supported flow
The critical-state model can explain not only the viscous-plastic behaviour of shales in structures such as shale anticlines and diapirs (Fig. 1c‒e), but also the initiation of the fluid-like behaviour of shales in mud volcanoes. At critical-state, all cements, bonds, and sedimentary and tectonics fabrics are destroyed by pervasive shearing, and the material flow is purely frictional (failure envelope has no cohesion and passes through the origin in a p′–q diagram). Thus, when pore pressure increases in mud volcanoes so much as to bring the effective stresses to zero, the grains lose contact, and the shear strength becomes zero (Figs. 3 and s2 in Supplementary Information 2). In this case, the Herschel–Bulkley behaviour converges to a viscous-fluid model. In this case, the shale is mobile even at small shear stresses, which corresponds to the behaviour of the fluidized73 shale in mud volcanoes (further details in Supplementary Information 2 and Fig. s6).
The drop in effective stress leading to fluidization of shales in mud volcanoes may result from an increase in fluid pressure or a drop in total confining stress. One important scenario producing fluidization occurs when mobile shales rise up a fracture system below a mud volcano74. Total confining stress decreases as the material approaches the surface. This drop in total stress during rise leads to gas phase exsolution (e.g., methane and CO2) in organic-rich shales, for example, elevating the pore pressure and promoting fracturing8, 22, 75. This combination can bring effective stresses to zero, leading to the highly fluidized ejecta sourced from mud volcanoes.
Brittle versus ductile behaviour
There is a longstanding debate concerning the relative importance of brittle and ductile behaviour in mobile shales4, 5, 7, 76. Brittle behaviour is seen in a stress–strain plot with a strain-softening behaviour; i.e., residual strength at the critical state is significantly lower than peak strength (Figs. 3b, 5a). Conversely, ductile behaviour is associated with strain hardening, having similar residual and peak strengths (Fig. 5). The critical-state model suggests that shales with either behaviour can reach the critical state (plateau at the end of the curves) and become mobile (Fig. 5a). In Fig. 3, for example, the sample reaches the critical state through brittle behaviour. A shale can behave in a brittle or ductile way, depending in part on confining stress, temperature, and strain rate42, 77, 78 (Figs. 5a,b and s3–s4 in Supplementary Information 2).
Where shales have reached the critical state through a brittle behaviour, any brittle structures formed during approach to the critical state may be preserved and severely reworked in lenses between anastomosing shear zones during critical-state flow, resulting in a mixture of structural styles5, 6, 15, 72.
Pore-fluid pressure
Low velocity of mobile shales suggests overpressure is high in these shales4, 7, 9,10,11, 13, 14, 30, 31, 45,46,51,52. The critical-state-model provides insight into the role that overpressure plays in shale mobilization. In principle, it is possible to mobilize shales at any pore pressure by purely increasing shear stress to reach the shear strength (see the critical state line in the p′–q diagram in Fig. 3b). However, increase in pore pressure decreases effective confining (mean) stress (p′), making it possible to reach critical state at lower shear stress. Without high overpressure, forces driving shear stress in mobile shales may not be enough to bring the shales to critical state and make them mobile. This explains the observed correlation between shale mobility and high pore pressures.
Several sources have been suggested to increase pore pressure in mobile shales, including disequilibrium compaction or generation of hydrocarbons48, 62, 47. The critical state model suggests shear-induced overpressure as another mechanism for increasing pore pressure in mobile shales as they deform toward critical state (cf. blue curve in Fig. 3c, and Figs. s2–s4 in Supplementary Information 2). This is because, under undrained conditions, any increase in either deviatoric or mean stress promotes rock compaction and as trapped fluids are incompressible, they must bear the load48, 47.
Degree of consolidation
The critical-state concept can explain mobilization of both consolidated and unconsolidated materials55,56,57, 59,60,61. Consolidated or cemented shales have a higher shear strength than unconsolidated materials, particularly when they are compressed parallel to the fabric or stratification79 (Figs. 6c and s5 in Supplementary Information 2). It thus takes higher shear stress to break cements and drive these shales to the critical state. However, once this disaggregation occurs, critical-state flow can occur just as in any other shale.
In contrast to previous interpretations13, critical-state mechanics suggests that there is no depth or temperature limit on the formation of mobile shales (Figs. 6 and s2–s4 in Supplementary Information 2), Even if cementation has occurred, mobilization in deep shales remains possible—it just takes a higher shear stress to overcome shale strength.
End of shale mobility
In many areas, stratal patterns on seismic data suggest that some formerly mobile shales became later inactive (Figs. 1c–e). The critical-state model provides an explanation for stabilization of formerly mobile shales. According to our model, mobile shales stop mobility when shear stress becomes smaller that the shear strength (Fig. s6 in Supplementary Information 2). This may occur through either a drop in shear stress (e.g., by the ending or lessening of the tectonic activity) or an increase in shear strength (e.g., by consolidation due to pore fluid drainage or cementation due to diagenetic transformations in shale); in either case the shale would depart from critical state (Figs. 3, 5). For example, in shales that have become mobile due to regional shortening, shear stresses may drop if shortening stops.
Seismic properties
The critical-state model also gives insight into seismic properties of mobile shales. First, it predicts that overpressure increases in mobile shales due to shear deformation48 (Figs. 2 and s2–s4 in Supplementary Information 2) and seismic velocity thus decreases, which affects the seismic impedance of the shales14. Second, shear stiffness of a material drops significantly approaching the critical state. Therefore, our model predicts that mobile shales should have a lower S-wave velocity than immobile shales at the same porosity30, 31, 81. Third, although flow at the critical state destroys preexisting rock fabrics, it may create new flow fabrics5, 6, 71, 72. Seismic anisotropy is therefore affected.
An important consideration in seismic imaging of mobile shales is whether the shales are presently mobile—that is, whether they are presently at the critical state. Once shales leave the critical state, overpressures and shale stiffness may return to normal values. However, any changes in anisotropy will remain.
To illustrate the application of the critical-state concept to a particular structure, Fig. 6 schematizes how the deformation would occur throughout the fold core of an anticline with mobile shales (Fig. 6b), in contrast to the same fold formed by slip along a discrete fault without any shale mobility (Fig. 6a). The overall fold geometry is the same, but the occurrence in the fold core of a penetrative deformation, the superposition of brittle and ductile structures, the abundance of tensional fractures and veins (formed possibly by hydraulic fracturing80; Fig. 6c), the notable inflation of the shale layer, together with the possible occurrence of extruded mud flows above the fold crest74, are indicative of mobile-shales deforming at critical state.
Conclusions
We propose that the critical-state model is a viable hypothesis for the mechanical behaviour of mobile shales. This proposal helps to answer many of the key questions related to mobile shales:
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Is deformation in mobile shales brittle, ductile, or both? Prior to reaching critical state and becoming mobile, shales can experience either brittle or ductile deformation. At critical state, flow in shales generates an anastomosing network of highly-sheared material, surrounding lenses in which previous structures are preserved. Thus, both brittle and ductile structures may exist in mobile shales.
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Can shales with diagenetic cements become mobile? Does depth of burial play a role in shale mobilization? We have demonstrated that there is not a diagenetic threshold for shale mobility. Deeper, more consolidated and cemented shales can achieve critical state and flow, although it takes higher shear stresses to break cements and drive these shales to the critical state.
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Do all mobile shales have to be highly overpressured? In theory, it is possible to reach the critical state purely through an increase in shear, without any increase in overpressure. However, critical state is much easier to reach if overpressures are present, because overpressures decrease the mean effective stress. This is consistent with information from seismic velocities, which suggest that most (if not all) mobile shales are overpressured.
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How does shale mobilization affect its seismic properties? The lack of internal seismic reflectivity in mobile shales is consistent with destructuration of the sedimentary and tectonic fabrics in shales due to large shear deformation and plastic flow associated with the critical state.
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Why do some mobile shales become immobile? Mobile shales stop mobility when the shear stress drops below the shear strength, and the shale departs from critical state. This may occur through either a drop in shear stress (by the ending or lessening of the load driving the shear stress in shales; e.g., by the end of tectonic activity) or an increase in shear strength (e.g., by consolidation due to pore fluid drainage or cementation due to diagenetic transformations).
We believe that this model offers many exciting prospects for future research. We look forward to seeing tests of this hypothesis as the study of mobile shales advances into the future.
Change history
31 January 2022
A Correction to this paper has been published: https://doi.org/10.1038/s41598-022-06170-2
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Acknowledgements
This paper has benefited greatly by the thorough and helpful comments and reviews by Chris Morley, Peter Connolly, and an anonymous reviewer. Andrea Billi is also thanked for his editorial support. We thank Nancy Cottington for initial figure drafting and Cari Breton for creating the map for Fig. 1. The project was funded by the Applied Geodynamics Laboratory (AGL) Industrial Associates program, comprising the following companies: Anadarko, Aramco Services, BHP Billiton, BP, CGG, Chevron, Condor, Ecopetrol, EMGS, ENI, ExxonMobil, Hess, Ion-GXT, Midland Valley, Murphy, Nexen USA, Noble, Petrobras, Petronas, PGS, Repsol, Rockfield, Shell, Spectrum, Equinor, Stone Energy, TGS, Total, WesternGeco, and Woodside (http://www.beg.utexas.edu/agl/sponsors). The ideas and content of this contribution have been strongly benefited from various fruitful discussions with colleagues such as Peter Flemings, Doug Jerolmack, Maria Nikolinakou, and Demian Saffer. Publication authorized by the Director, Bureau of Economic Geology, The University of Texas at Austin.
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Soto, J.I., Heidari, M. & Hudec, M.R. Proposal for a mechanical model of mobile shales. Sci Rep 11, 23785 (2021). https://doi.org/10.1038/s41598-021-02868-x
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DOI: https://doi.org/10.1038/s41598-021-02868-x
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