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Rocksalt is widely discussed within the scientific and the industrial world. The many
problems that salt diapirs give the exploration industry , the possibility of waste
disposal in salt mines, saltmining; there are so many sides to discuss. The possibility of
inviting speakers from this broad range of interest has been a real challenge. The program
that was finally created by the symposium organization therefore contains a large number
of speeches, all very different, but they all share one thing, namely Rocksalt.
I
There are two types of salt sequences, cyclic sequences with thinbeds of impure halite
deposited in ephemeral brines on shelves (like mostof the Zechstein of Europe) and thick
beds of pure salt that probablyformed in deep permanent brines in rifts (like Z2 or the
Louann of the Gulfof Mexico). The first type is rather stable, often undeformed
andcomparatively well known, the second is less stable and therefore common indiapirs but
has less well known origins.
Both types of salt sequence occur together in two regions of Iranwhich pro- vide well
exposed examples of two of the many current categoriesof secon- dary salt structures
recognised today.
Diapirs of Eocene-Oligocene salt exposed in central Iran areimpressive from the sky but
disappointing on the ground; this is becausethey circulated as they rose and are now like
ductile ice bergs and floatand spread beneath the surface of a ductile overburden. Whether
or not saltcirculation entrains country rocks can be very important to salt engineering.
By contrast, diapirs of Neoproterozoic-Cambrian salt in the Zagrosare much more impressive
on the ground because some extrude into the skyand spread like glaciers over a stiff
denser overburden. An overburden ofstiff carbonates delayed the buoyant rise of this salt
until Jurassic whenit began to flow into NS pillows above old faults jostling in
thePrecambrian base- ment. Reactive diapirs began surfacing long beforeNeoTethys closed
but surviving salt pillows deflated almost explosivelywhen overtaken by the serially
advancing front of the Zagros fold/thrustbelt.
Salt sheets in the sunshine of Iran are useful models of spreadingorogens as well as salt
sheets elsewhere. Direct measurements of their flowrates emp- hasise the potential
difficulties of exploiting hydrocarbonstrapped beneath examples in the Gulf of Mexico (and
the Red Sea, Khazakstan etc.)
amage to casing in salt formations can be caused by two distinct mechanisms: point
loading when poorly cemented washouts close, or by long term geological flow of salt in
regions of active salt tectonics.
We investigated the second mechanism, to quantify the risk of casing damage by flow of
salt in one of Shell's sub-salt prospects in the Gulf of Mexico. The approach was to
measure the in-situ strain rates of the salt body (in thin sections of rotary sidewall
samples), and use this data, in combination with a geological model to quantify the
velocity field of the salt. Our results indicate that excessive casing deformation by this
mechanism will not occur during the field's life. This can lead to substantial cost
savings by providing the basis for optimizing casing design in future wells in this field.
Structural style in the sediments overlying the salt in the area investigated indicate
large deformations of the salt during the Pleistocene, coupled with growth faulting and
incipient diapirism. However, the rate of growth on the faults has decayed with time, and
only two are active at present.Thin sections of rotary sidewall samples taken in a recent
well reveal a fully recrystallized microstucture diagnostic for large deformations, and
well-developed, equiaxed subgrains within the halite crystals, indicating that dislocation
creep was the main deformation mechanism. The subgrain microstructure was interpreted and
quantified using the petrographic workstation at Shell Research Rijswijk.
Then, using laboratory calibrations, we used the average subgrain diameter of each sample
to calculate the in-situ differential stress and strain rate in the salt during the past
few thousand years. This in turn was be translated into a velocity field using the model
of the flow field expected in the salt. The geological model predicts a general squeeze
flow, with only a minor horizontal movement of the top salt interface. As a best
approximation, we used a horizontal Poiseulle flow model to calculate the expected
horizontal displacement of the casing after 15 years.
Results show that the displacements are minor. A sensitivity analysis indicates that
significantly larger deformations are unlikely, and that the associated strains in the
casing will remain elastic. Results of the present study can be further substantiated by
refining the geological model using palinspastic reconstruction, and further quantifying
the strain rate versus depth profile in additional samples taken in a future well.
In order to gain quantitative insight into salt tectonic processes, and to model such
processes either numerically or using physical analogue techniques, appropriate data on
the flow properties of rocksalt and the underlying deformation mechanisms are needed. This
contribution examines the state-of-the-art in this field, focussing on experimental
studies, microstructural work and the implications for large scale tectonics.
In recent years, numerous experimental studies have been conducted on the steady state and
near steady state deformation behaviour of natural rocksalt under the (confined) low
temperature conditions (20-200 C) relevant to salt tectonics. Though variable, the results
essentially show that at stresses below 15 MPa and strain rates below 10-7s-1, flow is
dominated by dislocation creep processes, probably with cross- slip control at relatively
high strain rates, and climb control at low strain rates. Depending on water content,
these processes may be accompanied by fluid-assisted dynamic recrystallization. Various
creep laws are available which adequately describe the observed behaviour. Data on
subgrain size and recrystallized grainsize versus flow stress are also available for the
dislocation creep regime, with potential applications as paleostress and even in-situ
stress indicators.
In addition, recent experiments on fine- grained salts have demonstrated that when
sufficient water is present (e.g. > 0.05 wt%), deformation can occur by pressure
solution creep. Combining the flow law for this mechanism with those describing the
dislocation creep behaviour leads to a multi-mechanism creep equation and deformation map
for rocksalt. The latter suggests that under natural conditions flow will occur by climb
controlled creep or pressure solution, or by both. These mechanisms may also be important
with regard to a number of geotechnical problems.
Turning to microstructural studies, analysis layered, domal, intrusive and extrusive salts
shows evidence for the operation of both climb controlled dislocation creep and solution
transfer processes, sometimes simultaneously. It thus seems justified to apply the
laboratory flow laws and deformation map to model natural halokinesis. The results
obtained imply that the effective viscosity of salt during natural flow falls in the range
1016-1020 Pa.s, that pressure solution and dislocation creep probably alternate in
importance during diapirism, that buoyancy-driven pillow initiation can probably occur
only by pressure solution creep, and that initiation is more often triggered by
differential loading forces. Salt tectonics modelling studies also highlight the urgent
need for rheological data on sedimentary overburden rocks, and for data functionally
relating salt rheology and microstructure to water content.
Lubricating squeeze flow of a ductile salt substratum under varying overburden is a
characteristic element of the tectonics of many salt basins. Large-scale mass movements of
creeping salt can take place in this manner, accompanied by deformation of a sedimentary
overburden. A theoretical approach will be outlined, that attempts to capture the early
stages of this process up to the initiation of salt nappes and diapirs.
The theory assumes slowly varying overburden and substratum thicknesses. It treats the
salt (or shale) substratum as a viscous "lubricating layer" and the overburden
as an elastic "shear layer" that deforms by inhomogeneous simple shear along the
vertical. Assuming that the sea bottom (sedimentation boundary) and basement
paleotopographies are available from reconstructions, the theory can be cast in the form
of a single differential equation in the layer thickness of the salt layer.
In the presence of two or more creeping layers, the problem leads to a corresponding
number of coupled equations in the individual layer thicknesses, which must be solved
simultaneously. In the absence of significant lateral displacement of the overburden
across the salt substratum, the layer thickness is governed---in the simplest case---by
the a form of Reynolds' "lubrication equation" for a Newtonian viscous fluid.
The evolution of the salt layer thickness is coupled through the gradient of a flow
potential to the variation in overburden thickness and to the overburden's average
resistance against inhomogenous shear along vertical sections; if the latter is negligible
and if the densities of overburden and substratum are assumed to be equal, then the
driving potential gradient is proportional to an effective overburden gradient, while the
average flow rate in the substratum is proportional to the square of the layer thickness.
Layer thickness therefore strongly influences the growth rates of squeeze-flow structures.
Furthermore, squeeze flow in a substratum of varying thickness will force folds into the
overburden that travel in the direction of decreasing load with a speed proportional to
the overburden gradient. Buckling occurs in an elastic overburden under transverse loading
by a buoyant substratum only if sedimentation or erosion takes place. From the various
applications of the theory, the prediapiric stage in salt tectonics will be discussed as
an example.
The successful application of geophysical techniques hinges on the validity of
assumptions on subsurface characteristics. In reflection seismology, the application of
CDP stacking of traces may, strictly speaking, only be applied when the reflecting
surface(s) have no dip at all. Needless to say that this is rarely the case and ways have
been developed around this problem, resulting in good quality stacks in moderately
structured areas. Migration of seismic data is the process whereby diffracted energy is
brought back to its focus, thereby correctly positioning reflectors in space.
The application of most common algorithms for time migration gives satisfactory results in
areas with not too abrupt changes in lateral velocities.. However, in more than moderately
deformed areas, time migrations do not succeed in properly imaging the subsurface and the
industry had to look for a different approach.
There is no larger contrast thinkable than between the Dutch surface and its sub-surface.
Virtually the whole of sub-surface, comprising the Dutch part of the continental shelf,
exhibits a wild structuration with a large variety of structural styles. Very prominent
features are related to numerous saltdomes of all shapes and sizes, of Zechstein age. In
many places they cap Rotlie- gendes reservoir rocks, target of intensive gas exploration
efforts.
Seismic imaging of these Rotliegendes reservoirs is often hampered if not thwarted by
shape and nature of overlying saltdomes. Traditional seismic methods do often not yield
adequate results, essentially because of severe violations of assumptions as described
above. Prompted by a high level of exploration interest, novel methods are being developed
to improve seismic imaging below the salt. These range from a relative simple de- and re-
migration excercise to a full 3D Pre-Stack Depth Migration. Such PSDM at present
represents a very considerable effort, both in man-hours as well as in CPU time on
super-computers, with a total cost running in the millions of guilders for a few hundred
cubic kilometers of seismic data.
It should be clear that salt challenges the geophysicist of today to the utmost. Given the
very considerable efforts being spent at refining and accelerating PSDM algorithms, one
may expect that one day these results in its routine application throughout the reflection
seismology industry. If that would become true, the salt, now often considered a curse,
would ultimaltely prove to be a blessing also for geophysicists
Akzo Nobel is one of the world's largest producers of sodium chloride (salt).
Production methods include conventional deep mining of rocksalt, solution mining and solar
evaporation.
The Business Unit Salt Europe produces approx. 5mm tons of salt annually by means of
solution mining. Production facilities are located in The Netherlands (Hengelo and
Delfzijl), Germany and Denmark.
The presentation gives an overview of development and state of the art of present solution
mining techniques. All phases from exploration over development and production till
abandonment are covered whereas special attention is paid to the influence of numerical
leaching simulation and sonar surveying is outlined as well.
Mechanisms of subsidence and how to avoid respectively to monitor these are described
subsequently. Finally an outlook is given over future secundary use of solution mined
cavities e.g. for storage of natural gas.
Radioactive waste results from applications of radoactive materials in industry,
medical care, research in several fields and in nuclear power plants. An overview of the
annual waste stream will be presented.
The radioactivity of some categories of waste forms a potential risk for man and
environment for a very long time, up to many thousands of years, and consequently a
careful isolation is needed. As man made structures can guarantee isolation for a period
of several hundreds of years only, other solu- tions are required. Wordwide research is
going on whether desposal of the waste in carefully selected deep and stable geolgical
formations constitutes an isolation for a period long enough to allow the waste to decay
to an insignificant radiation level before entering the biosphere. An overview of the
different options will be given.
In the Netherlands the research was focused on disposal in salt formations. The
presentation of this research will be concentrated on the performance assessment, its
methodology, and main conclusions.
The analyses indicate that the expected exposure occurs in a far future and is very low.
It has also been foud, that the current properties of the overburden are important for the
minin, but less important with respect to the long term safety.
The presentation will conclude with an overview of the different discussion topics on
performance assessment of geologically disposed waste.