Held on March 20, 1996

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.

Interesting links about rocksalt:
Giovanni Guglielmo Jr’s research

Prof. dr. C.J. Talbot (Hans Ramberg Tectonic Laboratory, Uppsala University, Sweden): Two types of salt sequence & two types of salt structures

Dr. J.L. Urai (Shell Int. Exploration and Production B.V., Rijswijk) & Dr. T.N. Diggs (Shell Bellaire Technology Centre, Houston, Texas): Long term stability of wells in areas of active salt tectonics

Prof. dr. C.J. Spiers (HPT-laboratory, Faculty of Earth Sciences, Utrecht University): Microphysical aspects of the flow of rocksalt

Dr. F.K. Lehner (Shell Int. Exploration and Production B.V., Rijswijk): An approximate theory of substrata creep and contemporaneous overburden deformation

Drs. G. Diephuis (NAM, Assen): Salt as seen through the eyes of a geophysicist: a curse or a blessing in disguise?

Ir. W.A. Paar (Akzo-Nobel): Solution- mining

Dr. Ing. J. Prij (Netherlands Energy Research Foundation, Petten): Geological waste disposal: A downright solution !?

Two types of salt sequence & two types of salt structures
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.)

Long term stability of wells in areas of active salt tectonics.
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.

Microphysical aspects of the flow of rocksalt
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.

An approximate theory of substrata creep and contemporaneous overburden deformation
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.

Salt as seen through the eyes of a geophysicist: a curse or a blessing in disguise?
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

Solution-mining
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.

Geological waste disposal: A downright solution !?
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.

The symposium organisation:
R.A.J.J. Huijsmans Praeses
M. de Rooij Ab actis
A.A. de Ronde Fiscus
D.J.J. van Hinsbergen Vice Ab actis
M.W.H. van Hattum Assessor I
H.E. van de Kasteele Assessor II