X-Raying Salt Diapirs

The story of the research forming the basis for my fourth and final talk is based in: (i) a research project that went slightly awry; and (ii) an at least partly chance enc0unter I had with someone who I consider to be one of the greatest geologists who ever lived.

In 2008, Rob Gawthorpe (then at the University Manchester) and I landed a decent-sized research grant from Statoil. The associated project was to focus on salt-influenced rift basins (so-called ‘SIRBs’), focusing primarily on South Viking Graben and the Halten Terrace, both of which lie on the Norwegian Continental Shelf. We hired two PhD students and two Post-Doctoral Research Associates (PDRAs), loaded an ungodly amount of seismic and borehole data, and got to work. Within days we realised salt is hard.

salt
Salt tectonic structures formed due to flow of depositional (autochthonous) salt. Allochthonous salt structures, such as sheets (herein termed ‘tongues’) and canopies (amalgamated ‘sheets’), are also common. (from Jackson and Talbot, 1986).

Forget the depositional and lithological weirdness of salt, its mechanical properties are what truly sets it apart from other rocks. In essence, it has the ability to flow like a fluid, responding to differential pressures within the Earth’s crust (see image below). This odd behaviour causes salt to construct a bewildering array of truly giant (i.e. kilometre-scale), geometrically complex structures, including rollers, diapirs and sheets (see image above). One thing that helps the subsurface geoscientist is that salt is typically very well-expressed in seismic reflection data, being characterised by relatively high p-wave velocities (i.e. >4000 m/sec); as such, the top and base of salt-rich units are defined by high-amplitude reflections due to the encasing strata typically having lower velocities. However, when salt is ‘impure’ (i.e. contains clastics, carbonates, sulphates, such as anhydrite), thin, or has been mobilised and is thus highly strained, it may not be well-imaged in seismic reflection data. In the South Viking Graben (the Upper Permian Zechstein Supergroup) and the Halten Terrace (an unnamed Triassic salt-rich formation), largely pre-rift salt had a major influence on the tectono-stratigraphic development of the superposed rift systems. I was confused, scared, and with sponsors meetings coming up, a little nervous…

salt_properties
Left: graph showing the strength of salt with depth compared to other sedimentary rocks. Note that salt is: (i) weaker than other sedimentary rocks; and (ii) does not get stronger with depth, reflecting the fact it is largely incompressible and does not compact at it is buried. Right: graph showing the density of salt with depth compared to clastic rocks. Note that salt does not become denser with depth, being less dense than associated sedimentary rocks at depths >800 m. These and other properties mean that salt can flow and, given certain conditions, rise vertically. See Jackson and Hudec (2017) for details.

 

Clearly needing more training in salt tectonics, I decided to attend the Salt Tectonics Short Course at the AAPG ICE, which in 2008 was being held in Cape Town, South Africa. The course was being given by Martin Jackson, a Senior Research Scientist at the Bureau of Economic Geology (BEG), University of Texas at Austin. His research group, the Applied Geodynamics Laboratory, were and remain the world-leaders in the study of salt-tectonics. The course was excellent and I learnt a huge amount; it helped hugely with the SIRB project. However, the best thing about the course was bumping into Martin (OK, I was partly stalking him…) in one of the coffee breaks. I stopped short of asking him for his autograph, and instead told him about some of the salt-related work I was doing. We got on very well and, to cut a long story very, very short, he invited me to come and work with his group at BEG. Initially planning for a couple of months, in 2012 I eventually went to out to Austin for a year on sabbatical. It was absolutely ace and I can say, without reservation, it was the most informative and chastening year of my research career so far. Martin became a very close friend and I’m still sad at his passing last year.

In Austin I was pretty much given free reign to work on whatever I found interesting. Having dabbled with salt in the SIRB project, I wanted to go deeper. Not all that nasty business related to its deposition, oceanography and geochemistry (see the awesome book by John Warren here), but more about the crazy structures that form when it flows (see image above). More specifically, Martin, Mike Hudec, Tim Dooley and I recognised that, although the controls on the external morphology of natural salt diapirs are relatively well documented and understood, considerably less is known about their internal structure and kinematics for four key reasons: (i) well-exposed, natural salt diapirs are rare because halite, a key component of many salt structures, is highly soluble and dissolves, whereas anhydrite, upon contact with water, converts to gypsum, leaving a karstic soil or crust that masks the diapir’s internal structure; (ii) the internal structure of exposed diapirs can be strongly deformed by gravity spreading of salt extruding at the Earth’s surface; (iii) even where diapirs are well exposed at the Earth’s surface, such as in Iran’s Great Kavir, exposures are largely two-dimensional; and (iv) thick salt is typically acoustically transparent on seismic reflection data, and internal stratigraphic markers that record strain are typically poorly imaged. Despite this, some stunning, hugely inspiring, pioneering work on North Sea salt by a team from Aachan had just begun to show the contribution 3D seismic data could make to understanding intrasalt structure.

intrasalt.png
Left: aerial image of a horizontal slice (or ‘time-slice’) though a salt diapir, Great Kavir, Iran. Note the complex folds (often referred to as ‘curtain folds’) that formed due to inward flow of salt into and up the diapir stem, and the impure evaporites encircling a purer, halite-dominated core. Right: cross-section through a German salt diapir showing a range of complex folds formed due Rayleigh-Taylor, density-driven overturn during diapir rise. Although these examples are truly amazing, they are principally two-dimensional.

The Santos Basin, offshore Brazil with its high-quality 3D seismic reflection and borehole data provided us with a superb opportunity to document the variability in intrasalt structural style in natural salt diapirs. This reflects the fact that Aptian (Lower Cretaceous) salt contained within the basin, despite being halite-dominated, is locally impure, containing layers of anhydrite and carbonate. These layers are very reflective, having higher seismic velocities (i.e. >5000 m/sec) than encasing halite (i.e. >4000 m/sec); as such, they make excellent strain markers. Indeed, Fiduk and Rowan (2012) had just demonstrated that the Santos salt contained a bewildering array of complex structures; it was out contention that they had just scratched the surface and that there was plenty more meat left on the bone. If you ever meet me, feel free to ask me about the absolutely epic paper review experience associated with our work…

It’s not often you get to work on something truly new, which only very few have tried before. Nor is it common for a data type, in this case 3D seismic data, to truly allow us to see something we had never seen before, or had at least seen in such rich detail. This all sounds great, but the problem is you still have to a shit-load of work to map the structures, interrogate the well data, party on 6th Street, eat lots of BBQ, swim after work, etc, etc. In total, I spent around 6 months mapping the seismic data, line-by-line, piece-by-piece. Occasionally Martin would come in and point at a mistake on a map or an error in a calculation. I asked him to close the door on his way out. Then I’d cry.

fig.10-recumbant folds and sheets
Seismic profile illustrating complex internal structures within salt diapirs, Santos Basin, offshore SE Brazil. See Jackson et al. (2014; 2015). See also Dooley et al. (2015).

The effort was, I think, worth it. We found out, what we at least think, some very cool stuff (see paper links in caption to image above), and in the talk associated with this research, I will present said cool stuff. I will demonstrate that in the Santos Basin, a range of complex structures (e.g. allochthonous bodies, intrasalt feeder zones, breakout-related flaps) may be developed in salt diapirs, associated with Rayleigh-Taylor density overturn during diapir growth (see images above and below). I will argue that, although based on the Santos Basin, our kinematic model may be more generally applicable to other salt-bearing sedimentary basins.

saltmap
Left: time-structure map from seismic data showing external morphology of a large diapir, Santos Basin, offshore Brazil. Note the relatively flat top and smooth flanks. Centre: map showing intrasalt structures (i.e. structures within the diapir shown on the left). These complex structures (e.g. thrusts, overturned folds) are all ‘hidden’ within the morphologically ‘simple’ diapir. Right: map showing intrasalt feeders, sheets and canopies within the diapir shown on the left. All in all, this is pretty nuts. See Jackson et al. (2015) for more details.

Time permitting, I may also talk about some work we did in the Egersund Basin, offshore Norway. Here, seismic data suggest that a large, Zechstein Supergroup-sourced diapir has a welded stem, although borehole data indicate that the stem actually consists of an inner, c. 1500 m thick, halite-dominated zone, and an outer, c. 250 m thick, anhydrite-dominated ‘sheath’. I will argue that the anhydrite represents ‘lateral caprock’ that formed late in the basin history in response to the migration of NaCl-poor fluid up the margins of the diapir, and dissolution of halite. It’s pretty wild.

egb
Left: Cross-section through the Epsilon Diapir, Egersund Basin, offshore SW Norway. This is a co-rendered amplitude-variance image. Also shown is the path of the doomed, but scientifically awesome, 9/2-8S borehole. Centre: geological interpretation of the intrasalt structure and composition of the Epsilon Diapir, based on seismic and borehole data. Note the halite ‘core’ and the flanking anhydrite ‘sheath’. Right: data from the 9/2-8S borehole. The Jurassic shallow-marine reservoir flanking the diapir was the target of the borehole. It was wet, as in filled with water. See Jackson and Lewis (2012) for more details.

The results I will present here represent only the preliminary steps in improving our understanding of the internal structure and composition of salt bodies, providing some initial insights into the type of structures that may be encountered during drilling through thick salt. Furthermore, the complex intrasalt structures and lithological variations described here may cause relatively large, abrupt variations in subsurface velocity, thus posing a challenge to velocity model building and imaging of sub-salt and salt-flank exploration targets. We think some of this work might be important. Even if it’s not, we had fun doing it. That’s surely what counts, no?

Author: Christopher Aiden-Lee Jackson

I am Professor of Basin Analysis @imperialcollege. I ❤️ 🏃🏿, 🚴🏿 and @basinsIC (⛏). I obsess about the tectono-stratigraphic development of sedimentary basins. Why? Because I'm hopeless at everything else.

3 thoughts on “X-Raying Salt Diapirs”

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s