Author: Simon Blondel (OGS Trieste)

In a previous post, I tried to write a summary of the objectives and the benefits of studying Salt tectonics. In my next posts, I will try to provide an overview of the current state of knowledge in this domain and what the scientific community is working on: in other words, what’s hot in Salt tectonics? This is based on the “Salt Tectonics: Understanding Rocks that Flow” conference I attended, organized by the Geological Society in London last October 2019. I will detail 4 of the debated topics that retained my attention during the conference:

  • How does the salt flow and what is influencing it?
  • What are the parameters that govern salt tectonics and how do they interact?
  • What is the role of halokinesis in the structural and sedimentary development of a basin?
  • What are the limits of our models?

How does the salt flow and what is influencing it?

Answering this question is important because it determines what rules and equations we use to numerically model salt movement and predict the subsurface properties, and help us in assessing the objective trajectory well-path to mitigate shear stress of salt layers on its casing (Weijermars et al., 2014). Rather than studying the force driving salt flow, we investigate how the salt unit itself flows, under which physical laws and how to express the movement mathematically. Here, by salt unit we mean a halite-dominated salt unit that deforms viscously.

Three principal types of deformation regime have been defined for salt (Fossen, 2016; Sarkarinejad et al., 2018; see Figure 1 for a one-layer model):

  • the Poiseuille flow dominated by the pressure gradient and shear forces at its boundaries;
  • the Couette flow dominated by the shear stress;
  • a mixed of both (Hybrid flow).
Figure 1: Schematic diagram of flow in a salt channel of width h. Poiseuille flow with velocity profile caused by pressure gradient within the salt flowing into diapir (left), Couette flow with velocity profile caused by shearing within the salt layer beneath sliding block (center), and Combination of Poiseuille flow and Couette flow (right) (modified from Sarkarinejad et al., 2018).

Which is most important varies with the thickness and the composition of the salt unit as well as the dominant strain regime. The governing equations for these flows are known (special forms of the Navier–Stokes equation) and therefore they can be used to run our simulations of salt tectonics. Parameters that govern these equations include the density and viscosity of the mobile salt unit, as well as the pressure and the shear stress in the medium.

The problem is that at the scale we are working at, the laws governing salt movements can change. At the regional scale for example, the salt unit is considered as a single isotropic ductile layer (a one-layer model), primarily composed of halite, following a unique dominant flow regime. However, if you have the chance to go into the field, outcrops show that salt deposits are not just one big bloc of pure halite but often an alternance of halite dominated units interbedded with other evaporites (carbonates, gypsum, anhydrite), less-evaporite rocks or non-evaporite rocks (Figure 2). These interbeds will typically be more competent and brittle than halite, so they will not accommodate deformation in the same manner and often decrease the rate of lateral flow due to increased drag effects (Alsop et al., 1996).

Figure 2: Picture of halite layers within a salt mine. The orange arrows delimit the different layers of halite with different deformation features and colours. The deformation style depends on the lithology of the local dominant strain regime.

Considering these, you cannot just treat the salt unit as an isotropic one-layer anymore. Instead you must define an anisotropic multi-layered model where the strain is partitioned between the different layers, each of them deforming differently depending on its capacity to accommodate the local strain regime. The overall salt unit displays a first order flow, internal layers may exhibit a more variable second‐order flow (Figure 3). Typically, for the case of an overlying sliding overburden with a multi-layered salt unit, the salt layers above the basement will follow a Couette flow due to the induced shearing, but when the salt is not subjected to a gliding overburden the salt layers at the centre of the salt unit may follow a Poiseuille flow due to drag on the top salt surface.

Figure 3: Physical models simulating salt‐detached translation of a mechanically‐layered salt with (a) negligible roof and (b) thick roof showing a first‐order Couette flow profile and more variable second‐order flow; both of which evolve through time and become more complex and affected by Poiseuille flow as the overburden thickens (from Pichel et al., 2019; adapted from Weijermars et al., 2014). Viscous silicone polymer simulating salt (black) alternates with frictional‐plastic dry sand (thin, coloured layers, each 1 mm thick). Yellow parabolas represent originally vertical passive markers within the salt. For full model details, see Cartwright et al. (2012).

At the end of the day, what is defining the representative scale is the data. Only recently seismic data was able to image intra-salt features below the decametric scale. When present, these internal deformations can be used to understand salt flow within the basins and variations in deformation of the salt along dip and strike directions. But seismic is just an indirect indicator of the lithology and only through drilling we can directly assess the internal composition of a salt unit.

Current studies are focusing on imaging and mapping the intra-salt structures in order to better understand the kinematics of salt. This can be used to build multi-layered models that better represent the subsurface and better predict the subsurface physical properties. Presented work during the conference focused on:

  • the nature of the lithological control on intrasalt deformation (Sian Evans, Imperial College London),
  • new methods for calibrating kinematics of a salt sheet using fluid escape pipes (Chris Kirkham, University of Oxford)
  • the mobility of anhydrite, a non halite-dominated salt layers, when hydrated to gypsum (Johanna Heeb, Curtin University)
  • The composition of the salt and its variation using seismic attribute techniques to delineate areas of thin salt, areas of salt dominated by Layered Evaporite Sequence, and areas of thick halite (Shamik Bose, ExxonMobil Corporation)

As for us within SALTGIANT ETN, we are also working on the understanding of salt kinematics in work package 3 : Drilling hazards. I am working on the reprocessing of vintage data where I try to improve the image of salt structures and intrasalt reflectors when they are present. Part of Jimmy Moneron’s work should contribute to the understanding of strain partitioning within a multi-layered system of evaporites and clastics. Finally, Michael Dale aims to quantify the amount and distribution of excess pore pressure below and within the Salt Giant on the Mediterranean basin, and model the generation of overpressure by disequilibrium compaction, lateral movement of the salt, and mineral diagenesis at different Mediterranean geological environments from the Messinian to the present.  Wish us success and luck !


REFERENCES:

Alsop, G. I., Blundell, D. J. & Davlson, I. (eds), 1996, Salt Tectonics, Geological Society Special Publication No. 100, pp. 1-10.

Cartwright, J., Jackson, M., Dooley, T., & Higgins, S. (2012). Strain par-titioning in gravity‐driven shortening of a thick, multilayered evap-orite  sequence.  Geological  Society,  London,  Special  Publications, 363(1), 449–470. https ://doi.org/10.1144/SP363.21

Fossen, H., 2016. Structural Geology. Cambridge University Press.

Pichel, LM, A.‐L. Jackson, C, Peel, F, Dooley, TP. Base‐salt relief controls salt‐tectonic structural style, São Paulo Plateau, Santos Basin, Brazil. Basin Res. 2019; 00: 1– 32. https://doi.org/10.1111/bre.12375

Sarkarinejad, K., Sarshar, M.A., Adineh, S., 2018. Structural, micro-structural and kinematic analyses of channel flow in the Karmostaj salt diapir in the Zagros foreland folded belt, Fars province, Iran. J. Struct. Geol. 107, 109–131. https://doi.org/10.1016/j.jsg.2017.12.005

 Weijermars,  R.,  Jackson,  M.  P.,  &  Dooley,  T.  (2014).  Predicting  the  depth  of  viscous  stress  peaks  in  moving  salt  sheets:  Conceptual  framework  and  implications  for  drilling  viscous  stress  peaks  in  moving  salt  sheets.  AAPG  Bulletin, 98(5),  911–945.  https  ://doi.org/10.1306/09121 313044

Cover image from: https://earthobservatory.nasa.gov/

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